i found a simple schem of Line follower mobot using transistors.. the motors spin in the same direction(clockwise) when i build in Simulation..

basically the one motor must be spin in different direction(counter-clockwise) when placing it on the body of a mobot to move forward..

i'm littlebit confused with this.. what i did is i put the other motor in reverse polarity mode(- to Vcc and and + to the collector of transistor)
what happened to the motor i reverse is , it spin slowly than the other one.

 

Some motors are designed to spin well in only one direction.

When you connect the motor with reversed polarity then its diode must remain as shown on the schematic.