In debate, you rely mainly on the appeal of reason and facts. Some debaters approach the presentation of reason and fact through emotional deliver but this is different from trying to argue your point with emotionalism: what you don't want to do is rely on the appeal of emotional persuasion.
It depends on which type of debate category you choose. Are you involved in Lincoln-Douglass debate or public forum? If you want to practice Lincoln-Douglass debate, you need to be very familiar with the formalities of the category. Public forum--where you work with a partner--has fewer formalities. I would suggest visiting the National Forensic League's website for debate rules and topics.
One tip to keep in mind in any type of debate is your tone (your attitude). You need to appear confident and knowledgeable without coming across as belligerent and arrogant. The most effective way to defeat your opponent is to point out the flaws in his or her argument, not in him or her personally.
Finally, read, read, read! Stay up to date on current events but also read whatever is assigned you in your classes, whether it's history, literature, science, etc., you can always pull examples from a wide variety of subjects.
If you are not familiar with the formal rules of debate, here is a link that will be worth reading through. In preparing for a debate, you should be sure that you look at the topic from both sides; a common mistake that students make is only considering their own side of the topic, but if you are familiar with both sides you will be much better prepared with rebuttals.Also keep in mind that citing facts and statistics can enforce your position, but always double check them in advance and be prepared to give your sources.
Aside from being thoroughly familiar with all aspects of the topic you are going to debate, there are a couple of things that can help tip the scales in your favor. Either practice in front of a mirror, or take a video of yourself doing a presentation, and then really look at yourself. Pay attention to your body language, whether or not you make good eye contact, whether you make nervous gestures or use verbal fillers like "ummm" a lot. You want your body language to reinforce your words, not distract from them.