A great starting place would be to look for schools that have a 4 year degree in criminal law program. Other great undergraduate degrees that can help prepare you for an internship with the FBI are accounting, computer science, finance, another strategic business program. In addition, I would start to research and learn about federal and local laws. These knowledge based areas will all make you a great candidate for an FBI internship.
I would also keep in mind that you will likely need to pass a physical fitness test. These tests usually include speed, strength, and endurance components so it might be helpful to start building your fitness now.
Hope this helps!