What do I do if I fired due to appliying to the military?

Yes, you are right.  38 U.S.C. § 4311 forbids an employer from terminating an employee because the person has applied to join any branch of the military.  You can make a complain through the Department of Justice by filing a complaint withe the Department of Labor (DOL).  The DOL will investigate the complaint and if it believes it has merit, will attempt to resolve the complaint with your employer.  If that doesn't work, as the DOL to forward the complaint to the Department of Justice for review.  The Department of Justice is responsible for enforcing USERRA and if it takes your case it will serve as your attorney.  Or, alternatively, you can file a USERRA lawsuit with your own attorney, or on your own.