I'll guess you have a standard cup and cone system. See this site for instructions on how to adjust (scroll down to maybe the last 3/4 of the page):

http://www.parktool.com/blog/repair-...and-adjustment

You'll probably need a cone wrench (flatter than usual mechanic wrenches) that fits your cones, and a regular mechanic wrench for the locknuts. It's an easy procedure.