To pick out a fork, you need to know the following:
headtube diameter (if you had a fork installed previously, measure the outer diameter of the steerer tube to get this)
threaded or threadless (does the old fork have threads on the steerer tube?)
If threaded, headtube length so you can get the appropriate steerer tube length
crown height (determined by tire size and whether or not you want to mount fenders, not usually an issue with 26" forks)
brake type (cantilever/V-brake posts needed?)
rake (fork leg offset from steerer tube, not critical for casual cycling)
Judging by a quick Google search, your bike came with 26" wheels and was a MTB. In that case, you'll want a fork with cantilever/V-brake posts unless you are using disc brakes. Given that it was a 21 speed bike, it likely used a 1" threaded fork. All you should need to do then is measure your headttube and figure out what steerer tube length to buy.
There are plenty of cheap options here for forks:
http://www.niagaracycle.com/index.ph...er_id=&sort=3a or you can probably find something very cheap used.