First, having a 4 year degree in criminal law, accounting, computer science, finance, or something along those lines would be a start. Having an advanced degree could help your chances. Having knowledge of federal laws and or experience with Police work. Second I would recommend being physically fit and law abiding. Lastly, you can apply to intern with the FBI; however, some are unpaid internships.
If this is truly your goal, having the grit it takes to follow through will be essential. Get enough rest and self care as that will help you have the energy to get the work done. The FBI is a big life commitment. It will be important that you communicate openly and honestly to your significant other, friends, and family about the commitment it will take to be a successful FBI agent and to know your boundaries.
Sending you well wishes on achieving your goal.