I was looking for a way to reverse DC motor, and I though that transistors will be useful. Isn't it? If it's true, please can you give me a diagram for this. (I'm talking about reversing it automatically, without potentiometer or push buttons even if it's approximately the same ). Thanks!
Generally a gizmo called an H-bridge is used for reversing motors. There may be simpler setups just depending on what you want to do.
H-bridge is good, you can also do this with a DTDP relay. Here
The details depend on the voltage and current rating of the motor. And is it a standard DC motor?