As the first complicated bar move young gymnasts learn, the kip can also be your first experience of true frustration in gymnastics. It's also necessary -- the kip is a main bar stunt, used for mounts and transitions in early bar routines. Don't reconsider your gymnastics career just yet -- it takes most people weeks or months to get the kip. However, if you work on your strength separately in addition to practicing kips, you'll have a leg up on your classmates.
Even if you've done floor kips enough to have the feel of the motion, you'll never get the kip if you're not strong enough. It takes tremendous upper body strength to jam the bar after the pike, and only strong abs will allow you to end up on top of the bar instead of under it. Pushups, pullups and V-ups are vital because they work directly on the main kip muscles. Dips mimic the final push above the bar -- tilt forward to work your chest, then hang straight to work your triceps. Shoulder strength is also important, so do overhead presses and inverted pushups to help you jam harder.
A good, strong glide helps you start off your kip with momentum, and any competitive gymnast will tell you that momentum is half the battle. Stand at the bar and launch into a glide, reaching as far forward with your feet as possible. Keep your arms straight. When you get to the point where your body is almost parallel with the floor at the top of your glide, try it with a running start. This will teach you to catch the bar, and will force you to put some of those muscles to work as the extra momentum threatens to send you sailing across the room. Instead of letting go and falling, maintain your glide and transition into a pike.
You have all this momentum from your powerful glide, so use those abs to lift your feet as your body begins to swing backwards. Don't try to kip, just pike. Your body should be a triangle of all straight lines, with your arms straight, no sag in the torso, and your legs bent at the hip with your toes on the bar. Practice the sequence over and over until you're doing it so efficiently that you have momentum left over at the end of the back swing. If you have enough speed that your back swing raises you eye-level with the bar, move on to jamming.
The jam is where everyone runs into trouble. It's the first time a gymnast must rely solely on a few upper back and shoulder muscles to complete a stunt, and it can be daunting. This is where all of those pushups and pullups come in handy. Take a running start, glide, pike, then just as you begin to swing back, jam the bar from your toes to your hips. The motion is very similar to pulling up a pair of pants. The movement is simple, but the timing and strength are critical. If you allow yourself to swing back too far, you'll be at too wide an angle for the swing to carry you over the bar. Don't try to complete the kip, just try to end your swing with the bar at your hips. Once you can do this every time, it's a simple matter of maintaining your hollow and leaning forward, then pushing up to straight arms over the bar.