League has a very high learning curve, you can play thousands of games and still be learning, and that's usually part of the appeal.
As for where to start - I'll throw you some tips - I am a high platinum ranked support, so take it with a grain of salt:
1.) As for items, your early/beginner items should be about sustaining in lane, and maximizing your ability to get gold, as gold is power. You also need to take the matchup into consideration, if your going against an opponent who is going to poke you down, then your strategy might change.
Remember that every game is unique and there is no 'perfect' build - you need to examine whats going on in a match to decide what needs to be built. If you see they're mid lane AP burster is getting fed, then I would highly be looking at Magic Resist-style items that cater to my build.
As for lategame items? It can vary wildly. You develop a sense of what items work and what doesn't the more you play. Learn as you go. Watching popular streamers and looking up build guides CAN help, but part of the magic of league is finding out what works for you.
2.) As for what you should be doing? Generally, the early game is about farming and getting your first major item built. The jungler will attempt to roam and throw a wrench into those plans in the form of ganks. The early game generally ends when the first turret or two are down, this means farming is less safe, so champions will start roaming and grouping up to take objectives.
As you roll into the mid game, you'll see a variety of players doing a LOT of different things. Some will continue farming, trying to amass a big gold lead, some will split push, to try to pressure on a lane, and some will group up to try to co-operatively take objectives. All of these are good strategies, but its genuinely RARE (even at platinum levels) for all players to come together on this.
As for late game? you're trying to take an inhibitor Turret or Baron. Elder Dragon is usually secondary to all this. That's basically it, When it gets into late game, and simple mistake will cost you the game, so it turns into a high-stakes style game. Teamwork gets critical here, as dying means your out of the game for a LONG time.
3.) I would not recommend watching your own play until you have at MINIMUM at least 100 + games under your belt. You will not know enough to realize what went wrong. Instead, just play. In fact, let me give you some better advice.
**The single and most important thing for you to focus on right now is dying less. This teaches you everything you need to know. Did you die? Could it have been avoided? how. Answer those questions and you will start improving. You'll start examining your items (Would a Dorans shield have saved me?) (Why did I buy damage instead of defense as my first item?) (Trying to save my ally was a terrible mistake that got myself killed, next time i'll let him die alone, so the team suffers less!) Ect ect... After you develop a nice safe playstyle, the next steps are learning how to have more carry potential and impact in the game. You really start seeing players understanding this @ gold / plat levels. **
**As a goal - try to die no less than 6 times per game. Then 5 times. Always seek to improve **
Does this help you?
here
In overall enemy team has fed AD or many decent AD ? build armor
Always in a heat of battle with nearby surrounding enemies ? 
is a good active/passive
Enemies build lot's of armor against you ? well some armor penetration/lethality is nice.
e.t.c just use logic. The only base knowledge you need to know is what stat counters what. And just find items with needed stats, also paying attention on passives/actives.