Well, I'm just kinda starting out myself, I also ride Western normally, and I have learned how to jump from a freind ( I'm assuming you're pretty young too since you're in 4H! So am I! :P! I've never met another kid on here! :P)
So here's how I started, and I did the steps until Specs got them down good (like one step per week, about) And do NOT start out jumping him with you on him, rather lunge him so he can get used to the feeling before you ride him over them. Here's the steps I did, and I got help from a friend who is a jumper...
-Lunging on 4-5 groundpoles in a row
-Lunging on 3-4 groundpoles, then a small Xrail or cavaletti after them
-Lunging Alternating groundpole, cavaletti, groundpole, cavaletti, etc.
-Lunging 2 cavalettis in a row
-Lunging 1 small vertical
...THEN you can just repeat the steps above, except undersaddle instead of on the lunge line. Your horse should be pretty used to jumping by then and it shouldn't be a problem.
It totally worked for me, except I learned to jump with him with no prior experience, and no saddle... (I didn't own an English saddle at the time, but I do now! haha!)
Also, don't jump too high for your first vertical, and even though you lunged your horse over a vertical before, he still may wayy over jump it. That's what Specs still does often. Especially if it's your first jump of the day or your horse is really high strung... :/ I learned from experience.
Hope I helped!!!