Try using two potentiometers/ variable resisters, get them from speakers.
How ? Will u plz explain it ?Thanx for answer
You turn the potentiometer and resistance changes. http://en.wikipedia.org/wiki/Potentiometer
PWM. Use an Arduino.
What is Arduino Explain in detail !!!
I don't answer demands. Look it up on Google
+1 Yep added bonus by altering the speed of one motor you can steer it.