How do you fix your brake lights to turn on when you press the brake?

If one brake light isn't working the problem is probably a bulb that needs replacing. If both brake lights aren't working it is probably the fuse or the switch. Pull the fuse to the brake lights and replace it if it is blown.

If it is not blown get to the switch. Most cars have the switch located somewhere on the brake pedal. It generally has a plunger that is moved when the pedal is depressed. Unplug the switch ( with the key in the off position ) and use an ohm meter to test for continuity when the switch is in the position it would be in if the brake pedal was depressed. It should show continuity or open circuit ( in most cars ) in that position. If it is not malfunctioning see if it gets fully depressed when you push the brake pedal. If not adjust it so it does. If this doesn't work you have a short in a wire somewhere.

If it is malfunctioning, or if you don't have an ohm meter, take the switch out of the car, take it inside and carefully take it apart. Don't forget how it goes back together! Clean the parts inside that are metal ( probably copper ) of any corrosion and reassemble it. If this doesn't work you will have to buy a new switch.