Count the teeth on both of your rear sprockets. You will probably find 40 and 45. Using 45 gives more torque (hill climbing capability), but lower top speed. Using 40 gives less torque, but more top speed. You may not have to shorten or lengthen the chain as the wheel can move about an inch in the rear fork. If you have to shorten the chain, take out only one link. If you have to lengthen the chain, just add another master link.