There are several ways that you can prepare yourself for law school while you are an undergraduate.
First, if you have not met with your pre-law advisor, you should do so. Even if you do not have any specific questions, make an appointment and tell them that you are considering law school and want to know how to be on the right track. Your law school advisor will have a lot of information to give you. Don't cheat yourself out of this opportunity. If you have already graduated, many advisors will still meet with you.
Second, be sure to get a well-balanced education. Law school admissions committees are impressed by applicants who have a diverse and challenging course of study. Focus less on being a member of certain extracurricular groups and focus more on getting good grades. The latter carries substantially more weight than the former in the admissions process. There is no specific major for preparing law school. The best major is one that allows you to develop and display your academic strengths, which brings us to the final point.
Your undergraduate GPA is one of the most important factors in the admissions process. If you have not shown a strong academic performance thus far, and you still have one or two years left in school, now is the time to get on track. The worst thing you can do, grades-wise, is finish on a low note.