I know not to buy any runes yet, but should I buy champions? Or should I just be playing the free-to-play champions for the week? And only fill in roles that no body wants? So that I'll be able to get a broader experience.
Or should I call the role I want or play the champ I reeallly want to play? And focus on one champ
**TL;DR: ** Pre-30 : should I main a role/champ or just fill and play F2P?
play the free to play champs save for tier 3 runes and make sure to just get the standard ad and ap runes if you need to know them just ask. Wait till you have enough for your runes to start getting champions you that you like or it will make it rough for you against the players with runes
Buy all 450ip champions.
Fool around with them and choose one and a role you like the best. (Have a backup secondary champion/role)
Play almost nothing buy your main and backup.
Save all other IP for runes later on. Do not buy until level 20.
What this does: Testing F2P champions will only slow you down and confuse you. Not to mention you will spend so much IP you could have bought at least half a good rune pay. There are plenty of viable 450IP champions. As far as role: ALL top players have a role they like best and a champion they like best. Understanding a single champion and its mechanics is the best way to become competitive in this game.
Play f2p champs if you see 1/3 champs you really like and are good with buy them and save up for runes
If you notice you really like suporting and feel like your doing good with annie or tresh its usefull to always have them right
There are also some champs you can get for free like tristana garen and idk who but someone else
Play around with champions before getting them. Sample a little bit of everything and if there's something you like, get it. The first bit of playing this game is grinding for champions but the result is worth it.
In ranked, it's definitely possible to climb up the divisions by maining a champ but you won't always get your role. There was never anybody to tell me this so I've learned from experience that before level 20, you should ALWAYS save your ip to buy champions THAT YOU'VE PLAYED BEFORE. Meaning, test out champions that interest you whenever they're free before you actually buy them. I think filling roles is a really great way to become more experienced-- You should be able to play every role- top, jungle, mid, adc, and support- to a decent skill level (able to farm, poke/kite, not feed, general awareness, etc). Out of those five roles, you should have at least 2 mains and at least 2-3 main champs for each role. Unfortunately, I've grown accustomed to mid, maining LeBlanc so it's so difficult to win most of my ranked games, since LeBlanc is either always banned or I just don't get to play mid. You can buy runes either when you hit 20, or if you wish, 30. I started buying them after level 30 since I have a really small champion pool and wanted to increase it.
Play aram until 30. Then you get familiar with lots of kits and how to team fight correctly
I have spent like 120 dollars on champions and IP boosts for runes because 1. You cannot buy runes with RP, so you need to save your IP. Honestly if you have 20 bucks laying around you should definitely consider buying some RP. You can afford 2 975 champs and at least one of lower cost. Definitely worth it if you have the extra cash.
You can also save up IP to buy one champ that can singlehandedly carry you into higher ELOS (Hecarim Yasuo for example)
A level 30 player should know how to play each of the 5 roles competently. It's ok to have favorites, but no lv 30 player should be like "i can't jungle/support guys"
Start buying runes at level 20, and build 2 different rune pages. Physical and magic. Here's what works well for pretty much all champs:
Physical Rune Page: (tank, adc, most junglers) Reds-->Attack Damage Yellow-->Armor OR Health per level Blue-->Magic Resist OR Magic Resist per level Quint--> Attack Damage/Life Steal/ Attack speed (attack speed is good if you play adc a lot)
Magic Rune Page: (most mid/support) Reds-->magic penetration Yellow-->Health Per level Blue-->Magic Resist Quint--> Ability Power
Buy some champions that are easy to play and practice them. 
are pretty simple to learn.
ARAM is good for learning teamfights because its almost constant fighting. (Not much opportunity to learn how to CS though)
If you want to focus on one champion you can but I pref to focus on one champions when you are playing seriously.
Runes , start buying runes at level 20 and just build AD and AP runes I don't know that much about runes but I know that you get them at level 20.
You can play free-to-play champions if you want to cause when you reach level 30 you can get champions that you played and play them when you start playing ranked (if you know that you will do good.)
And you can play roles that no one wants so you can start seeing what you want to main when you are level 30.You can play the champion and roles that you really want also you can choose.
So yeah
Good Luck
