You will need a freewheel removal tool that matches the freewheel that's on the hub.

What brand of freewheel are they? If you look at the center of the freewheel where the axle goes through, you will see a mating surface for the tool. Possibly 2 notches, 4 notches, or a series of splines. That will help you identify what tool you will need.

Let Sheldon give you a hand here:

http://www.sheldonbrown.com/freewheels.html