If you've got a five-speed, you're looking for a 19mm bolt in the trans itself. It's about dead center on the front of the gear box, pointing straight back. It's the only one like it. If I were you, I'd drain the whole thing and refill it with GM Synchromesh, BG Synchroshift, or Redline MTL. It takes about three quarts to fill it. You'll know when it drains out of the fill plug.
If it's automatic, you're looking for a dipstick coming out of the tranny. Just get a funnel, stick it in the dipstick tube, and fill er up until that dipstick says it's full. Do this with the car idling in neutral.