Getting started with robotics is always tricky. For Raspberry Pi robotics, I always point people at the Pi Wars Hints and Tips guide. For micro:bit, however, you might find this guide a great place to start to build the robot pictured above. It uses a micro:bit, obviously, and the Kitronik Motor Controller together with two micro metal gear motors and a ball caster as the guts of the robot. It then uses a second micro:bit as the controller for the robot, operating over radio to issue commands to the receiver onboard the robot.
You can read the entire guide here and see a video of the robot working below: