How can I fix my bicycle to where I can change gears?

I think I have a 21 speed bike and the gears change perfectly on the left side of my handlebar from low to high, but i cant get the gears to change on the right side.How can I fix my bicycle to where I can change gears?
The various things that could be wrong, as you can tell by the other answer are extensive to explain. So, just go to this web site and print off the directions to fix your problem.



http://www.parktool.com/repair/How can I fix my bicycle to where I can change gears?
Your problem lies in your rear derailleur. This part is the thing at the back that your chain runs through with the two smaller gears. Its purpose is to shift gears.



Now, there could be a number of things wrong here.



Is the arm that holds the whole unit to the frame loose? Sometimes when you ride the bolt that holds it together becomes loose (no, it's not common, but it does happen). If it becomes loose, the arm moves. If it moves, it could fall off. If it manages to stay aboard, it certainly won't help you in shifting.

Check if the unit is loose by trying to move in forward and bike (from the bike's perspective). If the bottom arm swings but the mounting arm doesn't you're okay and your problem lies elsewhere. If the mounting arm does move, move it back (spin the unit CW) and tighten the thing down using an Alan key.



Another problem is that the cable that runs to the rear derailleur is loose. If it wasn't tightened down properly, then it could've come loose. If it's loose, the cable won't pull on the mechanism to shift the gear. On your rear shifters (this will be right side of the bike), try changing gears while standing next to the bike.



Does the derailleur move sideways? If not, check the cable. Does the cable move? If the cable moves and the derailleur doesn't, your cable is loose. You'll have to tighten it down (where do you tighten it? Put the gear shifter so that it'd be on first gear (smallest), and then put the chain on the smallest. Then tighten down the cable so it's tense). If the derailleur unit dos move, it's not in the cable problem.



Another possibility is that the unit just need adjustment (fine tuning). This is a long and tedious process, but it's easy. You'll find a number of screws on the rear derailleur. Three, I think (I know for sure two). Try playing around with these screws. They'll change how your derailleur moves when you put tension on the cable.



Still no luck?



Make sure the unit isn't broken or damaged. The arm may be bent and whatnot. If this is the case, you can try bending it back into shape. However, you'd probably just want to buy a new one. They're only 20$ (for a cheap one). The cheap ones do do the job. I have a cheap one (after I broke my old one) on my cheap bike. It hasn't failed. It gets me from A to B.



If the unit is fine, try finding a manual online that supports the specific derailleur you have.



Or, you could take a book out from the library that teaches you how to maintain and repair a bicycle. It'll include instructions that are more in-depth than mine on fixing derailleur issues. As well, there may be pictures (bonus!).



If all else fails, take it to the bike shop. If it's a simple problem they'll only charge you a few bucks. Worst case scenairo: you'll need a new derailleur. Cheapies are 20$, so don't worry about it.



Have fun!