Replacing a fluorescent light ballast is a project that can be completed within a half hour. You need a nut driver, a screw driver, a wire cutter, wire connectors and a new ballast.
Remove bulbs and take off cover
Make sure the power is not flowing to the fluorescent light, then take off the ballast cover. Some models require the use of a screwdriver or nut driver to remove this cover, while others are removable by hand. You should be able to see some of the internal wiring and the ballast itself at this point.
Cut all of the internal wiring
Using your wire cutters, cut the wires that lead into the ballast. Leave several inches from the end. Make sure you have cut all of the wires that connect to the old ballast.
Unscrew the ballast
Maintain a grip on the ballast while you use the nut driver to unscrew it. When it comes undone, it may fall to the ground, so be sure you have a good hold on it before you begin unscrewing it.
Mount the new ballast
Screw the new ballast into the old ballast's place. You may need to use a nut driver and the nuts that have just come from the old ballast to do this.
Redo the wiring
Examine the new ballast and the exposed wires. Match the correct colored wires and reattach them to the new ballast before putting the cover back on or the fixture will not work.