it's different for everyone :) have a trainer that you trust tell you when you're ready and help you out when you start.
Make sure you learn to jump with a coach or trainer. Also start slow, maybe start with trot and canter poles and practice your 2 point seat over these, treat them like real jumps thinking about your entry and exit from the jump. Also the jump isn't relaly your problem, it's the horses, you just have to worry about the line you take, your position, the pace your are approaching the jump at and the rhythm of the horse. If all these are good then you will prepared for anything the horse decides to do. If you fall off, as always get back on and try again, maybe lower to jumps or go back to poles on the ground but don't give up
When jumping make sure you never second guess yourself. You always have to have confidence in your horse and yourself. You know you're ready to start jumping when you feel that you are balanced and well trained on flattinh and poles! Good luck!
When going to the jump sit back and don't look down at the jump always look up and try and keep your heels down as much as you can overtop of the jump
As long as you are balanced in your walk, trot and canter and can completely control your horse