If you have a rusty nut and bolt, you can remove them by spraying penetrating oil liberally on the nut and bolt. Let the oil set for a while and then try to remove the nut. Penetrating oil can be purchased at your local hardware store. When trying to remove the nut, use a wrench with a long handle to provide you with good leverage. Push or pull the wrench with short and sharp jerks instead of extended pressure. The short jerks will help loosen the rust.
The phosphoric acid in cola drinks will cut through rust and help remove rusted bolts. This is a page about using Coca Cola for removing rusted bolts.