I attended a Master's course in Paralegal Studies. Not nearly as difficult as law school! What I found was that I did not know grammar - the various parts of speech became important! My recommendation is to learn grammar very well. Consider taking Latin, as there are a gazillion new words you will learn, many of which come from Latin. Learn to write a good paragraph , and from there, a good essay.
Consider everything available to develop self-confidence and public speaking skills, such as acting and speech. Learn to think critically, to be able to explain to others what you think and why, and to defend your opinion.
Beyond that, law is not just about research and writing. Every legal case is about something. For example, if someone is suing a contractor who installed his roof because the roof leaks, you might need to know a little about the construction trades. Or, if someone died during surgery, you might need to understand a little about medical. While at some point you may specialize, for now I think it is important to have a well-rounded background. This would include accounting, sciences, maths, etc. You might narrow your focus when in college, but try to be exposed to everything possible while in high school to help you determine what interests you the most.
This was an excellent question! I wish you the best!