You'll need to add some hardware, specifically a wheeled or tracked platform, to make your project mobile. You're going to use a platform that will use differential motion to propel and steer the vehicle. This simply means that instead of turning the wheels, you're going to vary the speed and direction of the two motors that drive the wheels or tracks. There are a lot of choices: some are completely assembled, while others require some assembly; or, you can buy the components and construct your own custom mobile platform.
Throughout this book, I'm going to assume that you don't want to do any soldering or mechanical machining yourself, so let's look at a couple of the more popular variants that are available completely assembled or that can be assembled with simple tools (a screwdriver and/or pliers). The simplest mobile platform is one that has two DC motors, each of which controls a single wheel with a small ball in the front or at the back. DC motors are...