Registration typically begins the first week of June and December for the Fall and Spring semesters respectively; the schedule of upcoming classes is published a couple of weeks prior. (You may be able to see the actual dates here.) Classes fill up quickly so you may want to consider volunteering, which is easy and will also save you money.
Most classes are 50-minutes long and meet for 12 weeks. Tuition is typically $120 for the entire semester (not a monthly charge). Volunteers, however, receive a 27% tuition discount and register early. See more details about volunteering and tuition here.