Mountain Biking - Noob Question - Suspension Fork!

I do not know anything about MTBs, just got a free unknown free from a friend and decided to build it up.

I would like to know how to determine what type of fork will fit in my head tube. The bike came with a Tioga Avenger headset, I believe that it is 1 inch - however the headset is much larger. Is this normal to have a head set that is about 1 1/4 with a 1 inch headset? Furthermore, can I stick as big of a fork that will fit in the hole?

Sorry if this is a stupid question...

From what I can tell, the Tioga Avenger is a "treaded" headset and from some quick searching, any suspension fork of recent manufacture is intended for use with a "threadless" headset.

It appears you might be able to find an older suspension fork but you have be concerned with the fork length as it will need to be cut and threaded to work with the Avenger headset.

My recommendation is that if you're going to be buying a new or recently manufactured used fork that you convert the headset to a threadless type. You'll need a new stem for your handlebars also.

Others might have better info and suggestions.

If you provided the make and model of the bike, we could more easily help.

Chances are that you have a threaded fork with a 1 1/8" steerer tube. Most MTBs use that diameter.
If you search Ebay for a threaded fork, the new fork needs to have that diameter, and enough length (measured from the fork crown to the top of the steerer).
It would make life easiest if the new steerer length matches the old one almost exactly.
Here's some info on headsets. (http://www.sheldonbrown.com/gloss_ha-i.html#headset)