For items start with Normal boots and 3 health pots, or, if you have no real mana regenration 2 health pots and 1 mana pot. You can skip the 2 doran's rings but it helps a lot in the early game with additional AP, health and mana regeneration. Then obviously take sorcerer's shoes. Than RoE, the mana, health and ability power are awesome. Afterwards take rabadon's deathcap, mage's best friend. Why do I take deathfire grasp? Well, the mana regeneration is awesome, it has some ability power and of course the cooldown reduction also helps alot. But that's not all. It's active part CAN be cast during channeling your ultimate. Than an abyssal scepter for the magic penetration. The archangel staff is good because you will have a lot of mana later in the game so this is a great pick. Than guardian angel for the resist and the revive capability.