Slowly! Start at a level you're comfortable with and do it over and over again until you could do it in your sleep. Then put the fences up a hole and do the same, until you're you're! I have a hard time with this. I used to be terrified of anything over 2'6", then it was 3', now 3' is a walk in the park but 3'6" leaves me running for the hills! It's a constant pursuit I find.
try your best to just have fun with your ride. start riding a horse you can trust - and ride them for awhile! i wouldn't suggest doing bounces and high jumps with a fairly new horse. once you trust the horse you ride try your best to just have fun with it. even if it's not perfect