Cyber security is currently the biggest growth area for the FBI. If you have the aptitude, consider computer science. Understand that there are many many different jobs inside the FBI other than being a special agent, including analysts and forensic accountants. As noted above, language skills are also useful - I’d guess Russian,Chinese and Spanish would be most useful.
Look up “critical languages” and “critical language flagship”. Alphabet agencies hire fol these.
For jobs other than special agent, the FBI will hire some 21 year olds who are right out of college. My son was recruited during his 7th semester of college,and given a “conditional offer”, but it took a while until he was authorized to start work. He had to go through lie detector tests, interviews, and an extensive security clearance, etc.