Keep your grades up as high as possible. Concentrate on courses that will give you an excellent command of the English language. In addition it would be beneficial to take speech and participate on a debate team if there is one at your school. If you think you might want to be a trial lawyer some experience in theater or drama will give you skills that are useful in a courtroom. Learning foreign language would also be useful.
Jeanette recommends the following next steps:
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!
Given you are still early in your high school career you have plenty of time and opportunity. In a few years I would suggest trying to get some internship experience at a law firm or in a corporate legal department. This is the best way to get a flavor for what it's like to be an attorney and may even give you an idea of what type of lawyer you want to be.
In the meantime, the best way to prepare yourself would be to get experience in some of the areas that make for a successful lawyer: public speaking and/or debate (especially if you want to go into litigation) and research and writing (English and History classes are great for this) are some examples. Otherwise, if you have any family or friends that are lawyers you can always reach out to them and ask them what it's like. If not, check with your school and see if they have anyone in their alumni network who is currently practicing law. They may even allow you to shadow them at their work so you can see what "a day in the life" is like.
Best of luck!