Adjusting the gears on your mountain bike is a relatively easy exercise that can not only make your rides more enjoyable, but also save you money since you will be doing your own tune-ups rather than having a bike shop do it. There are basically three different places you can adjust the shifting of your bicycle and this article discusses each one.
Find the "barrel adjuster" on your shifting mechanism (for the purpose of this article, the type of shifters does not matter. This information applies to both front and rear shifters). The barrel adjuster is a screw affixed to your shifters that loosens or tightens your shifter cable.
Loosen the barrel adjuster if you are having a hard time shifting to a lower gear. If you are having a hard time shifting up to a higher gear, tighten the barrel adjuster. Get your bike shifting as well as you can by using trial and error, then move on to the next step.
Find the barrel adjuster on your derailleur. Again, loosen the adjuster to shift down easier and tighten it to shift up easier. Again, using trial and error adjust your shifting until it is as good as possible. Using the barrel adjusters is the main way to adjust your gears.
Use the small adjustment screws located on the top of the derailleur to fine-tune any small glitches that may still be arising when you change gears. The screws will be labeled "Hi" and "Low." The "Hi" screw will help you shift up to a higher gear, and the "Low" screw will help you drop to a lower gear. Using the barrel adjusters and adjustment screws in unison is the only way to achieve a fully tuned mountain bike gear set.