Hi there
Motors generally do not need resistors to run unless you want to limit the current passing through them. Also for the kind of application you have specified above L293D seems to be the best option. If you want something which can adjust motor's speed then you should look forward to PWM concepts rather than this resistor thing.
I am sharing two useful links, hope these will help you:
1. "engineersgarage.com/contribution/dc-motor-speed-control-using-pwm-modulation"
2. "engineersgarage.com/contribution/line-follower-robot-using-8051-microcontroller"