Just do it?
Do you work with a trainer? A good trainer should be able to help you get started. Are you planning on showing? Do the shows you go to offer jumper classes? You can just enter into a class or two and start getting the feel of it. Obviously the courses and how you ride them will be a little different. I would suggest memorizing which "tables" are which so you know how to ride the course and when (or if) to do the jump off. For example, Table II Section 1A is timed first round, Table II Section2B is is timed first jump off, Table II Section 2C is Power and Speed, etc. There are a few more but I don't remember them all. Most local shows around here have only 1 or 2 different tables but the A shows will have a variety.