If it has a small hole on the side of the agitator near the bottom, insert a allen wrench to remove a set screw.
If it's older than that, I use an agitamer connected to a water hose, and slide it under the agitator, then turn on the water and sometimes it pop's off.
If stuck I smash at it with a hammer, then buy a new one.They have a bad habit on seizing on the shaft, that's why the later ones have a set screw.
But before you smash, it wait a day or so, someone else might have a easier way to remove it | 
Older Maytag Agitator Removal 
