How do you be confident when debating?

If you mean Debating as in arguing...

Try to come up with arguments that the other side might use, and find effective counter arguments for them. Have lots of evidence and be able to explain yourself clearly. Argue in fluent English and try to connect all your evidence after you've made your points. Make sure that everyone understands you clearly and explain all the facts.

Don't act like you don't want to be there.

Consider what Words you can and cannot use.

Avoid being outright rude.

Prepare.

Also, make every attempt to prove that you're right, but if you can't, then prove that they're wrong.