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 uses 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 come fully assembled while others require some assembly; alternatively you can buy the components and build your own custom mobile platform.
Let's look at a couple of the more popular units that come fully assembled or can be assembled with simple tools (a screwdriver or pliers). The simplest mobile platform is one that has two DC motors, with each motor controlling a single wheel. On the wheeled platform, there is a small wheel or ball in the front or at the back. Here is one example of a wheeled platform, available at many online electronics retailers:
