You definitely need a swiveling tripod that will support the weight and recoil of a rifle. As mentioned, standard tripods use a 1/4"- 20 threaded adaptor screw. If money is tight, you can tap ($6-8 bucks at the hardware store) a heavy duty steel scope ring, riser, etc (weaver or picatinny) and attach it to the tripod.
I'm bored so here are some photos of the aforementioned idea which should help for illustration purposes and this took me less than 10 minutes to do (had everything:
Edit: Take your time (don't drink 
) and center before drilling/tapping for aesthetic reasons.
