Pie pumpkin, about four pounds for a 5 gallon batch, preferably roasted. Alternately, canned pumpkin (be sure it has no preservatives) works just fine.
The first beer I brewed was Smashing Pumpkin from Northern Brewer. Used the kit and followed the instructions from their video for adding real pumpkin. Had to put a bit more spice in. It turned pretty ok. The NB video gives all the info you need.