Hi Brynna, awesome question first of all!! The FBI does really interesting work so major kudos to being interested in them. There are tons of different internships that provide good experience for a future career with the FBI, but the best internship to pursue is the FBI internship itself!! The FBI offers a couple different internship programs to college students and those internships are the best way to get a full time job!!
I actually have a few friends that interned with the FBI during college and they were all offered jobs at the end of their internship!! Not only is it the easiest way in, but the internship will also give you the security clearance necessary for full time employment. Getting the required security clearance necessary for the FBI is no easy feat so this is why they offer FBI interns job at the end of their internship (usually). With the security clearance, FBI experience, and the clear passion for FBI's mission, the FBI internship is the best way to getting full time employment with the Bureau.
I hope this helps and I wish you the best of luck!!