How can I completely reset android samsung galaxy s 4g? My phone has gotten extremely slow, and the GPS no longer seems to function which is the main reason I got the phone.

My phone is a Samsung Galaxy S 4G from T-Mobile.

Is it possible to reset it completely? I read several reset guides, and they said to hold the volume down + power and volume up + power but neither of those worked.

Thank you
I don't have that particular model, but every Android phone I've owned has had a "Factory Data Reset" option.  On my Nexus S, it's under Settings/Privacy.