Ummm seems like a dick answer but umm...buy some powersteering fluid and put it where it belongs, its late but i think its on the left side of the engine, if not the top right, just read the caps and it should tell you what you need, if not refer to your manual also.
Open the hood and there is a resovior for the fluid. Check your owners manual to see if the car uses power steering fluid or transmission fluid. Some nissans use transmission fluid instead of power steering fluid. I know the 300zx uses transmission fluid instead. Also Nissans are notorious for high pressure power steering hose leaks. You should look into that also.