In the beginning of the game, you do not want to fight. Focus on farming. You want to get a lot of stacks on your Q in the early part of the game. This is why your first items are focused on sustain. You should be able to just focus on farming in the beginning and have plenty of AP.

Now you should be level 6. This is when the fun begins. For a short time, you will just be slightly poking your lane opponent. Try to get him/her to about half health before engaging. You want to start by dropping your E on your opponent. Try to land the wall on your opponent, so that they are stunned for you combo and cannot fight back. In quick succession, you want to cast your W, Q, and R. When facing Yasuo, cast W, R, and then Q, as he sometimes comes out of the stun and throws his Windwall out before your R can hit him. Getting kills will also help you stack your Q, because part of your Q's passive is that any kills gotten on a champion, by any of your abilities or basic attacks will also grant you stacks. You max Q first, because when you have 5 points on it, you will get +5 AP from killing a champion.

Your focus late game is to stick with the team, or at least one member of your team, and roam with them. When you get into a teamfight, you will throw down you E, then use your W on all of them. Here is the important part. Decide, preferably before getting into a fight, who does the most damage on the enemy team. That is the person that you will cast your Q and R on, as no enemy, unless tanky, should survive that. I wouldn't recommend using your ult on the enemy team's tank, but there were times when he was the only person left alive on his team, or one of two alive, and I decided it would be worth it to try and kill him. Also, once you have full build and a good amount of stacks on your Q, you should be over 1000 AP.