All you want to do early game is stay back and farm your Q. I like soloing top as Nasus because it allows for uninterrupted Q farmage.

Use wither to help gank and don't forget to throw down your E when attacking champions since it reduces their armor.

At this point you should be absolutely trashing the other team. Your Q should be hitting ridiculously hard to the point where it causes rage quits (if you last hit minions with your Q throughout the entire game).