2001 pontiac aztek Has floor mounted shift. Won't shift out of park.

First of all, you are stepping on the brake pedal when you try to shift. Had to ask.

It could be a number of things. I have a 2000ish Grand Am that wouldn't shift. The problem turned out to be the T-handle on the shifter. Most GM shifter handles come off the same way. If there is a rubber/leather boot, pull that down a little to expose a metal clip.
The metal clip can be pulled off straight for\ward (I think), and the handle comes off.
The shifter will be a tube with a plastic rod sticking out of it. Try pushing the rod in, and see if the shifter works. If yes, replace the handle. If not, the problem is probably down in the shifter.

When you step on the brake, there is a cable going from the brake pedal to the shifter. Step on the brake and you should hear something moving in the center console/shifter. If not, the cable could be detached or broken.

The shifter itself may be broken, but that can be tough to get to.
The could be something wrong at the transmission, or inside the trans.