Summarize the following:
If there's a hip hop club in your area, you'll have to sign up.  Tell him/them you will sign everyone up. Make sure that you do not tell him what you are going to say in your rap routine! It's an improvisation anyway. Odds are you'll only be able to prepare in theory.  You can do it wherever you'd like, so long as you're not disturbing the peace. Right after school gets dismissed in the parking lot is a great idea if you want your battle to wind up on lots of cell phones or on YouTube. You may want to have your first couple of rap battles in the privacy of your own home. There's no shame in warming up. None of your peers have to know that you guys have been rehearsing for this moment. The only things you'll need are two people, a judge, and possibly some microphones or a quiet enough area. Decide a point system, such as best of three. Set a time limit for each round and a set number of rounds. What's more, decide who has to go first. It can be the loser of the previous week or find an unbiased way, like coin flipping. Going first isn't all bad. You get to cover your bases. If you know your pants are too baggy and your grades in math aren't exactly stellar, talk about it. That way your opponent won't be able to attack you as easily. If that's not an option, find a loop to play in the background. Take turns rapping insults at each other and generally showing off your creative, improv skills. Make it a rule that the judge will then name the winner of each round based on quality of insult and rap skill. Whoever gets the most points, wins. What's the prize? Apart from ultimate respect, of course.

Summary:
Challenge your friend(s) to a freestyle rap battle at the venue of your choosing. Have a rap battle with one friend, or a few friends. Have someone beatbox for you.