This compact, universal platform is great for your first steps into the world of robotics at a reasonable price.
The Amperka MiniQ platform is a constructor that includes:
The platform does not include electronics. In order for the platform to operate, it is worth buying control electronics, suhc as an Arduino, and a motor driver, i.e. a Motor Shield. In order to make it a bit smarter, you should add a few sensors and the corresponding logic to the microcontroller.
DC motors are not perfect and are slightly different from each other, even in one shipment. Use an encoder to control the direction of your robot for accurate measurement of distance traveled.
You can read about how to assemble a robot that can follow a line in this wiki article.