1) find a horse that is at your level and possibly one that you can advance with. 2) HUS and Jumping horses are costly. be prepared for the cost.
3) showing cost , besides the amt for each class, do have to pay for the trainer to be there ? how much ? charge for coaching each class? stall cost at the shows, rooms for you and parents, room for the trainer, food and Drink?
Compare horses and prices in your area. How often are you going to be out to ride ? how many lessons will you be taking? Farrier cost , extra feed supplements, blankets, saddles, bridles, pads, halters, leads, hoof dressing trailer wraps,A trailer ? and or the cost of paying to have your horse trailered to a show. I am sure there are many things I have not remembered to add in.