Typical AP Tank Build (for all AP Tanks)

reinforced boots for CC reduction and protections.

Breastplate of Valor for mana cdr and armor. (great early game)

Void Stone and Wall of Absolution for protections (buy based on what you need more first)

Midgardian Mail for hp armor and slows from auto attacks (works on mages auto attacks, also.)

magi's blessing for hp mana magic armor and a great passive.

Bulwark of Hope for heavy burst damage reduction magic armor and health

mystical mark for health armor and nice damage per second to everyone around you (better than it looks) can be replaced with Hide of the Nemean Lion

recommended
Sprint good for chasing or initiating a fight (tanks should initiate)
good to use early a fight to give you a huge advantage (make sure it hits most of your party members)

other
Meditationgood for mana hungry tanks.
good to use if you are getting bursted down too quickly (you are supposed to be focused)