Start small and don't rush into things with him/her. If you can, get a trainer to help you with the process! I've found that free jumping horses is always a good way to get them used to having to pick up their legs. :)
I'm currently training my own youngster to jump at the moment. I began lunging her over poles between jump wings to get her used to the jump wings and where the poles would be. Then once she was settled, I put out two poles between the jump wings on the ground so she could learn to pick up her feet and how to avoid knocking poles.
Only when she does not knock any poles and is paying attention to where she is placing her feet, I'd put up a small straight jump. It's expected that they knock it first time round (mine did a few times), but once they jump over the raised pole without knocking it, reward them with lots of praise. Treats in this situation work great as positive reinforcement, but I'd only advise using treats in a training situation if your horse fully respects you to avoid them crowding your space for food.
If your horse runs out (or refuses), simply redirect them towards the jump and try again. Remember, its a new thing for them. Getting frustrated will only make the situation worse.
Ensure your horse is certain about jumping on the ground on a lunge line (or free jumping if you prefer) before allowing them to jump with a rider. Jumping with a rider is a lot more difficult, since they have more weight to carry and have to get used to the riders jumping position and movements.
Good luck! :)