Make sure you stay above half health
Until lv.8 stay near the tower
Go for occasional jungles
If you do jungle keep exhaust ready
Assist your team when unoccupied
You are not a tank, do not act like one
You are a Mage, keep up your mana
Recall often to replenish yourself
"Go for the gold" farm some minions
No mercy, mercy is not a word in your vocabulary
"Playing stealthy is healthy" hit from the distance, don't go too deep alone
A wise man once told me,"hit where you can't be hit, and that is behind a freaking turret."
If the enemy is less than 25% health then chase the freaking sucker
Now get in good with your jungler, cause the other one is gonna rocket out of the jungle towards you
You have a need, a need for speed, run as freaking fast as you can
Lure your enemy out of cover
Throw wards in the brush

Look this build is very dangerous and messed up, but following it could help a lot. This makes you much more powerful than you counterpart late game. You may struggle a bit early game, but with some help from your jungler it will be alright. Just make sure you don't get poked down.

The build varies every game, it depends on who you are fighting, and how you play. This build is to keep you Alive and safe, not a blundering *********.