Purchase books on military law. Choose volumes based on both the military and the time period. There will be books focusing on the laws in place during specific wars, for instance, and the laws will change as societies evolve.
Contact law schools in your area to see if they offer classes or workshops on military law. The Texas Tech University School of Law, for example, established a Center for Military Law in 2004. Such organizations can direct you toward resources to determine whether you wish to invest more money and time studying military law.
Take courses toward a degree in political science. This could help you better understand the workings of the government and may complement your interest in military law. Take courses in military history and relations when possible.
Apply to law school. Contact the admissions office at the University and ask if they offer a focus on military law or perform a search on the website of your selected university for "military law." Some universities that don't offer a focus will still have student-run organizations that promote the understanding and knowledge of military law.
Subscribe to the Military Law Review, the U.S. Armed Forces journal of military legal scholarship. This periodical keeps readers abreast of American military law and policies.