Ya youre probably a bit low on tranny fluid.
Look at the side of your tranny case and youll notice two fairly large bolts. One accepts a 3/8 drive ratchet and is used for draining the case. The other accepts a 15mm or 17mm socket (could be wrong just thinking off top of my head) and is used for filling and checking fluid levels. Basically you just take the upper bolt off and stick your finger in there to see if the fluid is up to the hole. If not fill er up. Might also want to take a mirror and check it out and see what type of state the fluid is in.