High school debate is a very good thing to do to help you prepare to think like a lawyer. Debate helps you to think on your feet and that is a skill that lawyers need to have.
You can do a lot of other things too that will help. Take writing classes. Just as all good movies start with a good script, all good legal arguments start with a well-written brief or memo.
If you can learn to write well you are well on your way to becoming a good lawyer. When you write a paper, organize your thoughts by writing an outline of what you want to say and make sure that the order in which you say it makes logical sense. Learn the difference between fact and opinion. For example, it is my opinion that yellow is a pretty color. It is a fact that the sun is about 93 million miles from the earth. Listen to people talk and try to figure out whether what they are saying is a fact or is an opinion. Lawyers use both in presenting a case but it is important to know the difference.
Government classes will also help you prepare to be a lawyer.