Depending upon the type of spring, you shouldn't even try this yourself, rather call a trained professional or you could lose a hand or worse. If the spring is a "torsion" spring that sits above the door against the door header, then you should definately call a pro.
On the other hand, if your door is held with a series of steel cables, pullies and two springs, fixing these is fairly easy, if you have some mechanical inclinations. Without knowing what is specifically wrong, it would be hard to tell you what to do, or where to go for info, other than going to a garage door company and asking them. Stanley doors, among others should be willing to help, and will have the parts as well.
Repost with more info, or try the door guys. Good luck!