You can graduate with a bachelor's degree in any subject. Many pre-law students study political science, history, English and philosophy.
During your senior year of college, you are required to take the LSAT (law school admissions test) and make applications to law schools. The materials required with your applications are: transcripts, letters of recommendation, personal essays, LSAT scores and tax information for financial aid.
You must then attend an ABA-accredited law school for three years before you can take the bar. (There is an exception in California, where you can go to an unaccredited law school and still sit for the California bar.) During the first year, everyone takes the same courses and after that you choose your classes.
After law school graduation, you take the bar exam in the state you want to practice law. Once you pass, you are considered a licensed attorney. You can become a judge either by nomination from the legislature or by election; depending on your state rules.
Supreme Court justices are not required to be lawyers. Some states allow non-lawyers to become magistrates.