If you only wanted the motor to go one direction, then you could just tie an N-channel drain to VCC, the source to the pos. terminal of the motor, and the neg. terminal of the motor to ground. You could vary the speed by varying your PWM to the gate.
If you wanted the motor to go in two directions, and multiple speeds, that's where you need to encorporate 4 transistors in an H-bridge arrangement.
I suggest you check
this thread out for a good amount of info on H-bridges in general, and as applied to xmods. It has various links therein, which has a wealth of info on what an H-bridge is and how it works. Saves me alot of typing.
Also, check these sites.
http://www.dprg.org/tutorials/1998-04a/
http://www.barello.net/Papers/H-Bridge.pdf