In order to debate whether or not you should have homework, be prepared to lay out your argument in an organized, persuasive fashion and rebut the argument of the person you're debating the issue against. First build your case, and then point out the weak points of your opponent's case.
A debate is a structured argument in which you attempt to persuade the other party that your position on the issue is more convincing. Be prepared to make certain points, or contentions, and then support them with facts. If, for example, you're arguing you shouldn't have homework, your primary contention could be that you can pass the test without doing homework. Provide facts and examples to support your contention. A possible example would be a time in the past when you didn't do your homework and still got an A on the test.
Be prepared to answer, or rebut, your opponent's contentions. Your opponent may contend that practicing what you learned in class helps you to understand it better, that class time should be spent learning as much as possible or that homework is the time to practice what you've learned. Anticipate what your opponent might say ahead of time so you'll be prepared for your rebuttals.
Balance between presenting your own argument and rebutting your opponent's contentions. If you spend too much time focusing on what your opponent says, you won't be able to get your own points across. If you don't rebut your opponent's contentions, he will think you have no answer to them, so you need to do both.
Finally, as the debate is winding down, be prepared to offer a conclusion which sums up your argument. This should consist mostly of summarizing your own contentions, but also summarizing why your opponent's contentions are wrong. In your case, you could point out that you learn quickly and the time spent in class is sufficient to get an A on the test, and that the time you would spend practicing the material at home could be better spent learning to do something new.