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.