You're going to be a tanky Nasus but one who's got enough AP to do a lot of damage. You're probably going to want to go to the middle lane because it'll be a lot easier to solo farm and farm your Q.

Main Items are: Iceborn Gauntlet, Spirit of the Lizard Elder, Sunfire Cape, Hextech Gunblade, and Rabadon's Deathcap.
Iceborn Gauntlet will give you armor and AP.
Spirit of the Lizard Elder gives you extra damage, and even more damage to minions which puts them at an excellent point for you to use your Q.
Sunfire Cape gives you some more armor and health.
Hextech Gunblade gives you more AD and AP.
Rabadon's Deathcap will give you an immense amount of AP.

ALWAYS for Q every low health enemy! With your high AP + the high amount of extra damage, it can be devastating!
Spirit Fire can be used to lower minion's health to a point where Q'ing will be very easy. Spirit Fire can also be used to poke your enemy and keep him away from you while you're farming as well. When you're battling an enemy upfront, put Spirit Fire around you so they'll constantly be in the circle and taking damage.
Wither is very useful if you want to catch up to an enemy and hit them with a Q. Wither is also useful if you use it before Spirit Fire to keep the enemy in the area of effect longer.
Fury of the Sands works better if you use it right after the battle has initiated, because you'll have more health to keep and armor to back it up.

A balanced Mastery Set works very well with Nasus.