One of the easiest ways to get started building a robot is to purchase an off-the-shelf tank chassis. Some of these chassis even have the motors and motor controllers built in, or they are designed to work together, which makes it even easier to get started. The first robot that I built used the Rover 5 Tank Chassis and the Rover 5 Motor Controller Board, designed and manufactured by Dagu Electronics. Here is a photograph of my first robot:
All of the parts for this robot were bought including the top plate that the boards are connected too. Purchasing all of these parts can quickly get expensive, especially as you begin to add additional sensors and equipment to the robot.
If you are planning on building numerous robots, or even doing a lot of other prototyping with the Arduino, it is worth the money to purchase a 3D printer because rather than buying premade...