Kiwi - League of Legends

Normally you might see people using the same build in all games, this is very common in low Elo, this is good, but if you look at players like Wickd or HotshotGG, you will notice they always change either some item, or the build order, there is a reason for this, dynamic builds allow you to optimize your champion against your enemy and ally champion composition, a static build works, but it has a wider margin or error.
Some champions might be more versatile than others when it comes to possible builds and builds orders, but it's important to build according to your game.

The champions that almost never vary are AD carries and AP carries, AD carries normally build IE/Bloodthirster + Phantom Dancer and AP build Dorans + Deathcap or RoA + Deathcap, it's a rather expensive core, but they normally don't change much from game to game, after that they normally go for more damage, but you can consider getting something like a Quicksilver Sash or Banshees if you face, for example, Malzahar.

Some more dynamic roles are Bruiser, Jungler and to some extent Support.

Let's use Irelia as an example, let's say your core is Boots + Wits End, very cheap core and very versatile, you face GP in lane, you should consider starting with Cloth Armor + 5 pots, as the harass will be Parrrley spam, which is physical damage, with cloth armor, pots and armor runes you will do fine against him, you might consider doing the same with Garen, for example, Dorans shield and HP regen / 5 Quints also work nicely against him.

Now, you are still laning against GP, you are even in cs and it's time for you to come back, you get to choose between Mercury Threads and Ninja Tabi, if they have heavy CC Mercury is a nice choice, if they have heavy attack damage team, Ninja Tabi will be nice, so let's say you get Ninja Tabi, and Wits End, wards, pots and go back to lane, you kill GP and outfarm him, your team is doing nice overall, it's a good time to go offensive, you can build a Phage or Sheen, and rush a Trinity Force before getting tanky items.
Let's now suppose you did bad against GP, he killed you, he has a lot of damage with his Q, you should consider getting armor, an Atmas Impaler is a really nice choice, Phage + Atmas is nice, together with Ninja Tabi and armor runes you will have a lot of armor, and also, nice damage.
If you were laning against an AP, you and you think you will need MR, Mercury Threads + Wits End gives you a nice ammout, if you still need more, you can consider a Hexdrinker.

Irelia is a very nice example of a dynamic build, there are many possible builds for her, and building her right gives you a nice advantage.

Another thing you can do is change your build order, don't be afraid to get some extra HP or Armor.

Some common tips for building dynamically are:

• Try to have a cheap core
• If you and/or your team is doing well, build offensive
• If not, build safe
• See how professionals handle their build, seeing how Wickd itemizes Irelia is really nice (although sometimes he trolls)

Normally when you play LoL you don't really think on what to do, everything is pretty much automatic, last hit, harass, kill enemies, get towers, gank, control dragon, baron, teamfight and win, it's so automatic that getting out of that is what makes good players good.
HotshotGG said once something like "Every second you are not getting gold and/or experience you are wasting time", that really helped me be more useful to my team, get out of the automatic mode, and think what's better to do.

If you are a carry (not jungler or support) you must be always looking to farm, spend the most time possible farming, this doesn't mean to not b for a long time and get killed for that, but, everytime you b, check where you can go to farm, and when your lane partner is mia, of course you should say it but consider farm the time he spends away, this can give you some gold and level advantage, which will be really hard for your partner to fight against.
Say you are solo top, your enemy is mia, you suspect he will be ganking mid, you ping and warn your mid partner and keep farming, he fails his gank, and comes back top, by the time he comes back, you are already 15 cs ahead of him, that's like 300 gold, it's as if you killed him! He also lost experience. Sometimes though you have to follow him, it depends on the situation, but the point is, do something productive, if you gank and fail, you wasted time, gold, and experience.

Another important thing is knowing when to push, say you are solo top, and there are 4 bottom, the tower is at 1/4 hp and probably going down, it's surprising how many people try to go help in an already lost tower, what you should do is keep pushing, and try to get the tower, so it's one for one, or sometimes even better! Split pushing is really useful, but you have to know when to do it! When there are two missing it's normally a very nice time, if there are more only do it if you know you can escape, if you are someone like Shaco you are probably gonna be ok, or if you have teleport. Again, the point here is do something with your time.

Anyways, it's not always farming, once you have an advantage, you have to know how to use it, there comes a time when you teamfight, and you can't always leave your team 4v5, that's where experience is useful, you will know when to fight and when to push.

Another very common mistake is not knowing what to do when the enemy has baron, when they have baron you don't want to fight, your best shot is split push, push all the lanes the best you can and just defend, and don't get caught, getting caught cost games, specially if you are a carry, like Annie.

The idea overall is rather easy, just think "I don't have to waste any time, I have to always be doing something".

Again, this doesn't mean to farm top for 40 minutes, there are times when it's a good idea to gank, and you'll want to help your team with dragon sometimes, but this is experience, experience to know what to do and not waste time, if your enemy wastes too much time and you dont, you'll end up two levels ahead of him.

What you have to avoid, is beeing on auto-pilot mode, just not thinking, just farming and doing nothing else, see the map, see pushing oportunities, communicate with your team, suggest someone to push, this will really help you get some more Elo.

These are some basic rules which help lots to general gameplay.

1. Learn to farm, farming is really important, is the most secure and sustainable way of gettin gold, 80 cs by 10 minutes is ok.
2. One for one is not worth it, unless it's first blood. Normally you see many people diving to kill an enemy just to die after, this is bad.
3. Learn to harass. Poke the enemy when they are about to last hit, if you are ranged, works nicely.
4. Don't overextend if you don't have wards.
5. Try to control Dragon, get the timer, and ward it. If you are a jungler, getting a pink ward for Dragon is usually a good idea.
6. Tower kills are more important than champion kills.
7. Communicate, be a leader, say what you think your team should do, warn them if they overextend
8. If you want to win don't insult your team, you can rage all you want, just dont type until you are 100% sure you are gonna lose
9. Ward, get wards!
10. If you want to win, pick what your team needs, not what you feel like playing. Also be able to play all roles.
11. Know your champion, it can take several games to feel comfortable and use it nicely. If you can play two or three champions per role, and play them nicely, you'll be a great element for your team.
12. Don't get cocky, take every advantage you have, even when it's an easy game, take baron, dragons, steal their buffs, do everything, don't let anything to chance.
13. Normally when the enemy team get aced and your team pushes until they are all up, and all your team has to b, they will baron, don't let them baron.
14. When you find someone overextending in lategame, and you go to kill him, b afterwards immediatly, you will have all their team on you if you keep pushing.