all u have to do is play with the barrel adjuster. the parktool.com site is good. basically it's like this.
go to your rear tire. look on the right side. the chain goes through a thing that hangs off the side. that big thing is called the derailleur. follow the cable as it goes in and out. on one of the ends of the cable housing is a little barrel. twist that barrel counter-clockwise about half a turn.
sd