All you need to know
Rule 1: Farm more, die less. What this means is, don't worry about fighting early game. Just focus on farming to gain an early advantage.
Rule:2 Don't ult unless you need too. Your ult is there to pick off people that are getting away, NOT TO STEAL KILLS.
Rule 3: Stay in the back. You're a carry, not a tank.
Items after what I have posted are user choice. (including Crystalsys)
I have found that Mjollnir and Mask of Madness are an amazing combo and should be purchased 90% of the time.