You want to start with Doran's Blade - This will give u attack damage until u can buy a B.F Sword. U want Boots Of Speed to avoid damage of any caster. Once u have enough gold buy B.F Sword, this will give u 45 attack damage and will increase ur damage much. Vampiric Scepter will give u Lifesteal, this way u are harder to kill. Infinity Edge will give u 80 attack damage and 30 critical strike, so u will hit very hard once u reached that. The items after that, are important for damage, lifesteal and ofcoarse, survivability.

Use Flash properly, dont play with it. Use it to chase down enemy's or to run away.
Ignite will give u some extra damage that u will need to finish off people, ignite and Flash are a good combo to have, its damage + survivability