http://parktool.com/repair/readhowto.asp?id=105

This will show you how to adjust your hub bearings (assuming you have a cup and cone style hub). You'll need a cone wrench that matches the size of your rear hub's cones, along with another metric or adjustable wrench. If you need to fully overhaul the rear bearings (clean, replace bearings, and regrease), you'll also need either a cassette or freewheel remover tool depending on what your rear wheel has.