Your goal is to use the damage in the first minutes. Use your power and kill the enemy toplaner as fast as you can.
The choice of the form is up to you. I prefer Rhaast -> You will get some healing from damaging your enemies.
Your worst possible enemy is Irelia. She gets extremely strong after few minutes, but you can defeat her easily. Personally I just buy some armor and health items like deaths dance and goredrinker. This combination (especially armor) makes from Irelia an easy kill for you.
Speaking of runes, the best option is conqueror. It gives you bonus damage and that is exactly what you want, especially in early game.