How lucky you are to know this at such a young age! It will give you the opportunity to pay attention to certain things in school, that I did not think were necessary. Grammar! Learn the parts of speech, and the punctuation rules. Really learn them! There have been cases decided over the meaning of a sentence with or without a comma in it!
Beyond that, do everything you can to have a broad vocabulary, and to learn the root parts, as there is a lot of Latin in law. Challenge yourself every way you can to think, write, and speak better than you ever thought you could. Consider getting into acting, as well as debate and speech.
As you are going through all this, learn how to study, especially when it is something that does not interest you. Learn what works best for you! You want to be able to study both thoroughly and quickly. You won't always have time to read a chapter over and over and over again - especially in law school! Learn to take good notes when the teacher is speaking, and to make outlines summarizing lectures and readings. Learn to skim a chapter before reading it - look at the intro paragraph, all the section headings, and the conclusion -knowing what is coming makes it easier to read for comprehension!
Also work on developing good time management skills. Do not put off doing things just because you don't want to do them, or because they are difficult. Get those things knocked out first, and everything else will be easier.
I'm sure others have other ideas, but I hope I have covered most of the basics. It is important to have a solid academic foundation before heading into college, and law school!
best of luck to you!