Do you mean two-point? If you mean two-point while jumping, you want to wait for the horse to close your hip angle. I've found that it helps to close your eyes before the jump, if it's a small one, so you can't anticipate it at all. I'm sure other people will have suggestions as well :). If you mean half-seat, there really is no correct time for this (early or late, I mean). Usually in equitation, most people will ride a half-seat canter around the course. In other disciplines, too, of course :). I THINK you mean two-point, but if you do mean half-seat other people can give you more suggestions :).
Yeah, half seat is what you do before a jump out on the XC so you don't sit on the horse's back.
Just do your half seat on the ground to work on solidifing your leg. Then work on the lunge without your reins and work over low jumps. That will help you learn how to feel the jump. MIE and Spyder and Eventerdrew might also have some more ideas!