A note of correction: You would not be a lawyer IN the Court but rather a lawyer admitted to practice before the Court.
The first and most important thing you would need to do is finish high school and college and then law school. Your emphasis in College should be International Relations with a minor in a foreign language, preferably French. In Law School, you should pursue as many courses in International Law as are offered. Once you have graduated from Law School, you would need to pass the Bar Examination in the state where you reside, and thereafter apply for admission to all Federal Courts, including the United States Supreme Court. Thereafter, you should apply for an internship to work with the Court at the link provided below. That experience would be a healthy addition to your resume should you apply to a firm which handles international criminal matters.