When I put my headlights on, my brake lights don't turn on what could be the probblem?

I guess you mean you turn on the lights and the rear lights don't come on too.Did you check the fuse? If fuses ok turn on the engine and have somebody check your rear lights to see if the turn signals work,the brake lights too. If they don't work either then you may have an electrical problem that you can diagnose with a simple circuit tester (any hardware store -$10. or so) I can't imagine all the rear light bulbs went at once,