The vast majority of mountain bikes sold these days include forks with suspension. Suspension forks allow between 3 and 10 inches of vertical compression, making it possible for the forks to absorb the shock created by the terrain that you ride on. There are a few different types of suspension forks out there, and the repair methods will vary slightly. Frequent maintenance to your forks can sometimes mean the difference between needing a repair and needing a replacement.
Find out what type of mountain bike forks you have. Forks without suspension that have serious problems such as a bend or crack in the metal will need replacement, not repair. Suspension forks are commonly of two varieties, with springs used for compression and oil used for damping or air used for both compression and damping. The damping mechanism--or spring effect--is usually adjustable, either by external controls or by adding more air or oil, or thicker oil, in order to maintain the proper amount of vertical travel in your suspension.
Adjust your suspension forks to your weight and style of riding first, the external controls. External controls are in place to change the geometry and feel of the forks as you ride, meaning that sometimes when your forks feel as if there is something wrong, a repair is unnecessary and only an adjustment with the external controls is required. External damping controls are commonly located at the bottom of the each fork blade, while tension adjusters are commonly located at the top and are used to dictate the amount of travel within the fork's suspension.
Overhaul your suspension forks entirely if you notice that your forks slam through their travel too quickly and without resistance or they begin to feel loose, sluggish or uneven and creaky. Remove the front brakes, wheel and any other component attaching your forks to the rest of the bike. Remove any clips or screws that attach the blades of the forks to the lower (fatter) tubes of the forks. Remove the blades from the lower tubes slowly; otherwise you risk spraying oil all over the place.
Drain all of the oil out of the fork tubes and clean all of the parts thoroughly. Use clean oil to replace what oil was lost and add the amount suggested by the fork's user manual. Apply grease to the screws that hold your forks in position and loosely secure them with your hands. Use a screwdriver (or Allen wrench as the case may be) to firmly secure all of the screws without over-tightening or stripping them.