To the point of speed, this would be one, of many possible, ways to do it.
In this (untested) code your pot being in the center would stop motion. Go left and the motor goes one direction. Go right and the motor goes the other direction. The more you come from the center position the faster the motor will go.
// 10 bit input ranges 0 to 1023
// 8 bit output ranges 0 to 255
potVal = analogRead(potPin); // read the value from the potentiometer into potVal
if ( potVal > 520) { // if pot is turned to the right
motor.setSpeed(map(potVal, 520, 1023, 0, 255)); // make the speed proportional to the amount of turn on the pot
motor.run(FORWARD); // set motor to move "forward"
}
else if (potVal < 504) { // if pot is turned to the left
motor.setSpeed(map(potVal, 0, 504, 0, 255)); // make the speed proportional to the amount of turn of the pot
motor.run(BACKWARD); // set motor to move "backward"
}
else { // pot value must be between 504 and 520 which is our deadband
motor.run(RELEASE); // stop motor
}