Rap along. Memorize the words to your favorite rap song and rap along in your ear buds, in your stereo, in your car, etc. Do it loud, and do it with confidence! Try to rap along until you have every word memorized and (more importantly) you can hit all the beats correctly.
See if you can find the instrumental track of the rap song that you've got memorized. If not, find one that's similar. You can download from many places online. Practice the verses you've memorized over the instrumental beat. Again, do your best to stay on-beat. This will help you learn how to keep a rhythm and tempo.[3]
Once you're able to perform the rap song that you've memorized fairly consistently over the instrumental beat, try to adapt it to another beat. Pick one with a different sound and possibly a different tempo. Again, you can find rap beats online in many places. The point here is that you are working on becoming adaptive to the music that you are rapping on.