Prep: The prep is what helps get the jump off the ground. There are several types of preps used in cheerleading today. I will describe the prep that I use, and my cheerleading squad uses. Start with feet together, and hands by your side. Quickly bring your arms into a clap. Then pull your arms up by your ears. At the same time go up onto your tippy toes. This motion should be done fast, and with a lot of power. Next, swing your arms in front of your body. As your arms come down cross one over the other. Bend your knees at the same time. Be sure to stay tight throughout this motion. Now, swing your arms back up, lifting through your legs. Concentrate on pushing off the ground. Use your toes to get extra height.
Jump: When in the air keep your head up, and back straight. If you look down, the rest of your body will follow! Keep your toes pointed at all times! This is very important for the visual appeal of the jump. Do not reach for your toes in jumps. If you touch your toes, it will stop you from getting hyper-extended jumps. Instead reach for the inside of your shoe. Many judges rather see great form over height with bad form. Keep that in mind when you are practicing. Always remember FORM OVER HEIGHT! If you use the correct form NOW, it will be easy to keep the form when your jumps are higher!
Landing: Landing seems to be one part of jumping that people do not put a lot of emphasis on. However, the landing is a VERY important part of the jump. Without a great landing, the whole jump can look awful! WHIP your legs shut when coming down from a jump. Use the same if not more power as you did going into the jump. Do NOT float down from a jump. This could result in the famous "legs apart" landings! When hitting the ground absorb the pressure through the balls of your feet. Avoid landing flat footed! If you hear a LOUD thump when landing, most likely you are doing it wrong! I have seen 200 pound guys land softer then some 100 pound girls. So it is all in the way you land. Also, when landing bend your knees to absorb some of the shock. Do NOT land with your knees locked. This is very dangerous.
Now, just try it. If at first you don't succeed, get up and try again! Trust me, from personal experience, your first jumps will look horrible, but you will progress with time.