You didn't give us much to go on. What kind of controller are you using? An old mechanical speed controller doesn't have brakes, so reversing the control just reverses the polarity on the motor. Electronic speed controllers come in a myriad of types. Reverse isn't allowed in most races, so many controllers either don't have reverse or have a way to disable it. If it is disabled then you just need to turn it back on. The means of doing that depends on your controller. Sometimes it is a jumper, sometimes you program with a set button on the unit, and sometimes you need a programming card. If your car does have reverse, reverse won't engage until after you've used the brakes. It is a two step process to avoid going directly from forward to reverse. First you push forward once to put on the brakes, then you release the trigger and push again to go into reverse.
