First and foremost, learn to do this :
https://www.youtube.com/watch?v=prb47MkDX98
https://www.youtube.com/watch?v=sNpdezx8ep0
It'll take time but you'll really benefit.
Practice it during laning phase, since all you do is attack minions anyway, so might as well practice it then.
LANING :
So in lane what you wanna do is farm and harass.
Farming is pretty simple, you click the minion about to die, and get the gold.
Harassing is a bit tougher - basically what you wanna do is find ways to hit the enemies without taking damage yourself and without losing creeps.
So the way to do it is you watch your own minion HP. When your minion gets low, go a little in front. The enemy AD will come forward to take the creep, and at that moment you auto attack once or twice and back off to farm your own creeps. If you do this like 10 times, that's 10 autos maybe which is easily 70-80% of their HP. So when your creep is about to die, you get ready to attack the enemy AD.
And any time the enemy support is in range, land a couple of shots on him too.
Here's a more detailed video series explaining how to lane :
https://www.youtube.com/watch?v=rnE6tiHElSc
You also need to know how and when to back :
- Back when you have gold for your item. For example, don't back with 1350g for no reason. Wait 200g for the BF.
- Back when you are really low.
- Before you back, shove the wave into enemy turret if possible. This means the turret will kill all your allied minions, and the enemy minions will come unscathed into lane(so you save a bunch of minions).
- If the enemies back, shove the wave. This time what happens is since they backed, if you shove the wave in, the turret kills all the minions and they don't get any of the gold and exp.
Here's a good video :
https://www.youtube.com/watch?v=Tc63DWeIr3Q
TEAMFIGHTS :
Stand in the back of the fight, always try to attack the closest target that will keep you safe.
So each fight you have to figure out who can kill you, and where you should stand to prevent that. Once you've found out where to stand, you need to find the closest target you can and just attack him. Try to stay alive for as long as possible, even if it means people sometimes walk away from you with low health. That means 90% of the time you will be standing far in the back, attacking tanks, cause trying to go in for a carry means their tanks/assassins/bruisers will wreck you.
Your job is to deal as many auto attacks as you can, and the only way you can do that is by standing at the back, not dying and always auto attacking.
Couple of good videos about teamfighting :
https://www.youtube.com/watch?v=7DkXUcKDrcs
https://www.youtube.com/watch?v=G50boEuYeVo
ITEMS :
There's no point standing in the back and attacking tanks if you deal no damage to them.
To do damage to tanks, you will need last whisper 3rd item almost every time. If you're behind, get it even earlier.
Get PD if ahead. Get Statikk if behind.
On most AD's you want IE first item. And in general you want IE pretty much every game on every ADC.
Try to find out good builds for each ADC (pretty much the same with a couple of exceptions).