Mana per level runes for better manaregeneration in early game.
The rest explains itself.

Stack Dorans Ring for more health and enough manaregeneration through the game.
Buy Boots of Mobility to catch up with your enemys or switch fast between lanes to gank top or bot.
Stack ability power with Rabadons Deathcap and go for Lich Bane for a meele burst after your
Q + E combo.
Get a Void Staff if they stack magic resist otherwise head directly for Zhonyas Ring.

Last hit minions with your auto attacks in early game. Activate your E skill if your enemy wants to harass you while last hitting.
Later on you E skill is strong enough to clear whole minion waves.

Early Game:
Play defensive until you bought your boots. After that harass your opponent by activating your shield, stunning the enemy and popping your shield to cause crazy damage.

Mid Game:
Gank bot and top lane after clearing your minion wave as fast as possible. You are very fast with your mobility boots.

End Game:
Go on like before.
Focus The enemy carry is very import.
Do not waste your skills on the tank!