The first phase of a kip is the glide. It can be done either in a pike or straddle position. The glide must be powerful enough so you'll have the momentum for the rest of the trick. One effective drill here is to do 3 - 5 glide swings in a row. Remember: keep your feet low near the ground and open your shoulders at the front of the glide. (This will also strengthen your abdominal muscles over time.)
Pulling your toes to the bar is tricky when it comes to timing. You must have extended your body as far forward as your momentum will take you, and you should pull your toes in as your body is just beginning to swing back. This requires strong stomach muscles, so doing leg lifts is the remedial exercise here.
The last stage is even more tricky. It, too, requires the right timing and strength, and bad habits often develop here. You may have heard people telling you this is where you "pull up your pants," and that is essentially the movement you are doing with your arms. Your hands, which are still gripping the bar, will be traveling up the length of your legs up to your hips. At the same, you will be pulling down on the bar while your swing is rotating you up and forward.
This "pulling down" motion is dependent on the strength in your triceps. A horribly difficult muscle to work, I know! Along with doing push-ups and pull-ups to strengthen your triceps, you can do an exercise illustrated on the right (I don't know the name of this drill).
Stand on a set of panel mats, in front of the low bar. Begin with your knees bent half-way. Jump up and using your arms, pull down on the bar so that you land in a front support. This exercises is best done with your arms locked straight. Adjust the height of the mats or block or whatever you are standing on so that this isn't too easy or too hard. Once you get the hang of this, you can do them repeatedly in sets of 10 or 15.
As gymnasts work on their kips, they often have a tendency to pick up bad habits as they try to "force" themselves to make the kip. One of these is bending the arms and allowing the bar to come away from your legs. Another is kicking the legs back to try getting the momentum to get yourself upright. But you'll notice that this will pull your body away from the bar.
This is one of those skills that take a lot of practice to get. It will take a long time to learn the correct timing and to develop the necessary strength. Even worse, it is also one of those skills that you can lose shortly after learning. It definitely ranks as one of the most frustrating skills in gymnastics, but be patient. It will come to you soon. Good luck!