I am playing lol for a month and I've been copy pasting mobafire builds.But a lot of the youtubers have said that you should build according to the situation and the enemy team.I am currently playing top lane(garen,jax,darius)and I want to know which items to buy according to the situation.
Most champs have a no-brainer rush item.
Darius/Garen have Black Cleaver Edit: I guess Triforce Darius is a thing too
Jax has Triforce
After that it changes a lot. Let's look for items for all 3 champions:
Your standard boots when you think that the other boots are not needed.
Against heavy CC comps (excluding knock-up cc).
Against heavy slow comps and you think you are/will be tanky enough
When enemy has more than 1 critter or the enemy crit - ADC is a problem.
When you are the only front-liner in your team and have to lead the charge
/ 
When you want to splitpush (Isn't optimal on Garen)
Against heavy healing comps
An early bramble buy helps you against a healing top laner (For example Maokai or Aatrox) or against a ranged poke guy (Like GP or Quinn)
Against HEAVY AP comps. This item doesn't work with Steraks Cage so you either get that or the other.
You can buy the mini-version of Maw in laning phase when having problems facing an AP Poke champ (Like Lissandra or Swain)
Against Magic Damage based marksmen or heavy DOT damage or spammy magic champions like Ryze (
are the Magic damage based marksmen, though keep in mind Varus and Kai'sa can go an AD route, so watch their build)
When facing heavy poke comps
Against AD attack-speed based comps (do not buy this item coupled with Thornamil since your HP-Pool will be too low then)
When you opt into wave-manipulation / map control (Isn't an optimal item on neither of the 3 champions but it isn't the worst thing)
When you are going the tank route and want some wave-clear and bonus damage
Not that popular on the 3 champs but if you opt into a fast engage then this item will make it more consistent
Darius:
When you get kited too well and have problems closing up on people
A solid Darius item to negate burst damage
Your standard MR item since it bumps up your healing
Gives you movementspeed if you need it (In it's current state it's not that good of an item tho)
When you are ahead and/or your team cannot afford to lose you that quickly for whatever reason (not built often)
Jax:
An early Tiamat helps with pushing the wave very efficiently and gaining priority in laning phase if needed (mostly build as a first item even before your rush item)
When you are against way too many tanks and you DIDN'T pick Conqueror (Do not buy this when you have Conq)
Standard Jax item. Mostly bought after Triforce or as the 3rd item.
Solid item when you opt into split-pushing (Is a bad item on Darius and Garen tho)
When you are ahead and have problems sticking on people
Very situational item, it gives you extra sustain and empoweres your magic damage plus it gives you extra sticking potential and burst damage.
When you get kited too much and need life-steal (gives some burst damage)
When you have no problems sticking onto people and you are going a damage heavy or on-hit build
When you REALLY can't buy Triforce for some reason or you are going full-tank (this item doesn't work with Triforce)
Against heavy AP comps
A solid aggressive item for Jax
Garen:
Again, If you need the movementspeed, if not, ignore this item.
For when you are ahead (do not buy this when behind)
When you need the movementspeed and don't need tankiness (also gives higher roaming potential)
When you need the slow and ambush people a lot + you want vision control (do not buy this with youmus since your tankiness will not be enough)
After your damage items you pretty much go full-tank (normally you have 2 damage items like 
followed by 3 tanks items, you can go for 3 damage and 2 tank for more damage of course, getting more damage items after that isn't recommended since you will not be tanky enough to get that damage off in the first place)
Hope I was of assistance
Edit: I am no Garen / Darius / Jax main so I might have forgotten some very situational stuff
When ahead, build defensive. When behind, build offensive.
This rule isn't everything though, but it's something that can definitely help you when unsure.
Depends on what you are up against, what runes you are running, what your team has.
if you are up against a tank or bruiser, this is a solid first item - or if there's just several tanky champs on the enemy team
great for darius because it amps his healing - as long as they have some ap damage, this is a great buy
good early armor item especially if you are ahead; let's you run opponents down in lane and engage teamfights
great armor item vs auto attack opponents - especially crit ones
replace cleaver with this vs enemy comps where they are all squishy or at least 4 are; combo with sterak's
great if you can afford a second damage heavy item - especially good synergy with triforce
if you are snowballing ahead (strong/lead), this gives some nice damage and while still increasing your sustain.
your goal is all about extended fighting to proc your passive and look to dunk people
Heyo, here is a bit more advice:
First have a basic understanding about what each champion does and general counterbuild items. Generally they fall into anywhere from 1-3 of 5 categories: Tank, AD ability based, AD attack speed based, AP, and Utility. Also worth mentioning that it is important to build around their WHOLE team, not just your lane opponent.
If you are playing as a tank your goal is to build 4-5 tank items and either (vs CC) or (vs AD/AD attack speed) If you are having issues KILLING a tanky enemy team then as an AP champion you build 
&/or 
AD champs build and AD attack speed build Enchanters can build an 
to help their ADC burn through tanks more quickly.
AD ability based champion build anywhere from 1-5 damage items depending on whether they are someone like Darius or an assassin like Zed. They each tend to have a core item like Nelogis was talking about. Countering these champion depends more on playstyle than specific items, although flat armour helps a lot. aren't especially amazing as they don't focus on auto attacks. It is also worth noting that only helps vs champions that are building crit.
AD attack speed based champions tend to go for either on hits or crits. These champs are usually ADC's although there are a couple exceptions 
. Their builds tend to vary a lot from champ to champ (Mobafire isn't a bad thing) so I wont talk about their core items here, instead pay attention to the notes about how they can build vs enemies. Building vs them either means trying to 100-0 them as a mage/assassin or building to negate their damage. Crit champions don't like and pure attack speed on hit champs don't like .
AP champs can also vary a lot in build and playstyle, but they all tend to be shut down very hard by large amounts of magic resist and health. is love, is life. helps a lot vs spam mages like 
and 
is a great item for laning vs an AP champ (unless you are Garen) since it will convert damage into mana and mana use into health which helps a ton vs poke. Be careful of mages with though, it will provide a LOT of damage vs Magic Resist stacking tanks.
Enchanters are strange and there aren't really any items that counter them unless they are heavy healers like Soraka which is what I am going to talk about next. This is due to enchanters existing because of their abilities while their items just enhance what they naturally do.
These are the 3 finished items in the game that can cut ALL forms of healing in half. Generally tanks are going to want thornmail, while AD casters/AD attack speed want Executioner's calling and Mages/Enchanters(if no one else buys it) want a Morellonomicon. That being said a number of tanks have the option of picking up a mortal reminder such as Garen and Darius if they want to be able to proactively cut healing in half instead of only doing it for a very short time upon getting auto attacked (Good vs things like Soraka and Mundo)
Generally if anyone is ever healing a lot buy one of these, or even just the components 
both of which still cut healing by 1/2 but don't offer the rest of the stats/passives of their upgraded items. Remember that it applies to things like lifesteal too.
Think about the weaknesses each defensive stat has: Armour/Magic resist: True damage/penetration Health: %health Healing: Grievous Wounds
and the counter to each offensive stat: AD: Armour AP: Magic Resist True Damage: Health %health: Same as AD/AP %health true damage: Crying (Technically if you are someone like Mundo then regeneration is the correct answer here)
Pay attention to the types of items your champion builds and when you need to build a health item think about whether you want it to have armour, magic resist, or just a ton of flat health. For example: As a Jax who wants some more attack speed vs a heavy AP team a is a really strong buy, especially if your team does a decent amount of magic damage. That is what it means to counterbuild, wits isn't an item Jax builds frequently, but vs or with Heavy AP it is really strong, and can even be built as a first item in lane vs the likes of Swain.
Counterbuilding is rarely about abandoning your normal build, instead it is about swapping a couple items out for items that do similar things, but are more effective vs and with different kinds of champions.
the ultimate 
build for any situation
with summoner spell 
You simply smite and exhaust people and pile on the % damage reductions
then you laugh your balls off as the 6 item ADC can't even outdamage your base hp regen
don't use mobafire its fucking horrible also id suggest sticking to jax and darius over garen. champions usually have a core item or a few so or darius its usually black cleaver or deaths dance and jax with trinity n steraks, after that if you are laning against something like yasuo or tryndamere tabis and randuins is probably a good choice, if you are laning against something like swain and they have an ap mid / supp or jungle adaptive helm is probably good.
Champion.gg
Filter for plat+
Order might not always be "the best", but you'll get a good idea of what you'll need.
Learn champion kits and what they scale with, as well as the state of the game. Enemy team comp mostly physical OR enemy ADC fed? Buy Armor. Enemy AP carry fed? MR. But first, buy for your lane opponent. If you're ahead/winning trades, build damage. If you're losing fights, build defensively (HP + armor/MR). Enemy team have a lot of hard CC? Buy Merc boots. Just learn what each champion can do and what items' purposes are for.
Oh thats an out dated thing from when Both damage items and tank items were good. Damage items are better while tank items have been nerfed several times just go damage and never group with your team unless you an adc
ignore what these people say. being told what and why doesn't make you better. you have to think. getting used to thinking will help you improve. ask yourself these questions:
• what does this item do • why would i need this effect • will this item fulfill my needs for this game
i'll give one example
What does this item do?
it gives magic resist and reduces CC
Why would i need this effect?
if the enemies have a lot of magic damage and/or CC
Will this item fulfill my needs for this game?
look at the enemy team and what they do, then decide for yourself
that's the thing about builds. there is never one answer to the question "what should i build?" because it's part of the game you're playing to figure out what item is best for the situation you're in, mostly because no 2 games are ever gonna be the same, and things are always getting patched in and out.
BUT unless you're building into on 
for his first 3, then you're a scrublord and that's just the way it is
Depends on several factors (i.e. champion, scalings, ratios, point in the game, runes, etc) that you'll pick up om the more experience you gain playing and experimenting with the game.
Since you've only been playing a month, I would highly recommend just testing out different champions, items, and build-paths yourself in customs, while using the recommended builds when playing with others.
I see a lot of people mentioning basics, like get armor against a team with no magic damage dealers, get merc treads against teams with lots of cc, etc. But sometimes "build according to the situation" just means to change which items you build before others.
If your mobafire build says that you're going to get triforce first, then blade of the ruined king, then randuin's omen on jax; sometimes even if you get fed it might be better to get randuin's omen second. If you're crushing your lane, but mid lane yasuo is up two kills or vayne is on a killing spree, getting armor first will help smooth out team fights once you start grouping at 15-20 minutes.
You copy/paste korean builds.
I just bust open probuilds most of the time
Most champions have two or 3 core items, after that its up to you and the situation you're in.
I love Diana for her flexibility on that account. She can build anything AP related first, probably wants a deathcap and lichbane at some point, but the rest is open as long as it has AP. You can rush literally anything on her, whether it be Zonya's vs Zed, Banshees vs LB, gunblade vs akali/kat, nashors tooth vs Yasuo. Anything is fine as long as it has AP and fits the situation :)
