I would look at the links kreutz posted. Inexpensive to play with would be http://www.pminmo.com/ . I did this about 15 years ago.. started out with 4 transisters driving each of the phases of a unipolar motor. Then mosfets - then when I wasn't getting the speed I wanted.. current limit resister.. then found some chips that would do pwm.. Still only doing half stepping.
Finally we found some compumotor oem 650 micro stepping drives on ebay. All I can say is we never went back. Microstepping style drives are worth every penny.
Now if your doing it just to learn.. then have fun.