butt to the back of the saddle, and don't jump for your horse, keep your heels down and you're all set :)
If you look down, you go down ;)
Ok so for starts, the jump of the horses problem, you as a rider need to focus on the lines you take before and after the jump, the pace you approach it at, your position, and the speed that you are travelling to it. If you get all those right, everything should fall into place. When starting out it's best having an instructor with you to help your position or give you advice. Also if you are worries about a jump, don't jump it, wait until you are ready and have more confidence and then try again. For your position make sure you keep your shoulders back when approaching the jump and let the horse jump up first, the fold into your jumping position with your heels down, eyes up, hands forward and the rest of your body will follow. I would suggest jump with a neck strap - this has saved me on many occasions and I rarely jump without, just use an old stirrup leader and have it loose of the horses neck about where you will put your hands and hold onto it as you jump and when you land so you don't pull back on the horses mouth. Also after the jump don't be in a rush to sit up back into your normal riding position, let the horse relax after the jump, give him a pat then take your your position again. Hope something in there helps you and happy jumping!