First, see this thread:
http://www.bikeforums.net/showthread...bikes-to-SS-FG

As far as ID'ing freewheel vs. freehub: Take the wheel off and look around the axle end and see if there's a hole with splines, ala the Sheldon Brown link. If there is, it's a freewheel and you can just take it off and screw on an ISO thread (16t+) BMX/single speed freewheel. If you're lucky, it will line up with your inner chainring.

If the freewheel was made before 1984, it requires a hard-to-find tool (see pictures in the thread above.) Your best bet is to take it into a bike shop and have them remove it. Otherwise, you can get a standard Shimano freewheel tool to remove the freewheel that's on there, then screw the new SS freewheel on by hand.