I take Hextech Gunblade for the good damage,life steal and spell vamp.Nashor's tooth is very good item for atack speed and AP.Phantom dance will give you very good atack speed and movement speed,so you can run and join battle faster.Taking rabadon is good way to get AP.Rylai septer is for a little bit AP and good HP.You cant fight without good HP.

I make this build cause when you use alpha strike it will deal tons of damage.Your basic atacks will be good on damage and crit too.Your Meditate spell will heal you much with 513 AP :)
You can make easy double kill in 2vs1 cause of your alpha strike and your cool ult :D

This runes are very good for the beggining.It's cool to have more mana,atack speed and AP in the beggining.