If you would like to start building robots, then you probably should start with small ones. This way you will get results faster and much cheaper. As a starting point it is always best to start with line followers. Once you get it right, then you can move to more complex robots. But it seems that with small line following robots fun never ends. Roborizeh have been testing really small (~1.5cm) size robots that are driven with vibrator motors – same as used in cell phones.
When robots get smaller, casual driving systems may be challenging and sometimes inefficient. Small test robot is clocked with AVR Attiny45 microcontroller which has an IR sensor to detect line. Two cell phone motors moves robot forward. There is a needle attached to motors that makes it possible to move. Robot is powered with 3.7V Li-Pol battery. Motors are driven directly by microcontroller, as they don’t require high current. I think this is really cheap and fun solution for anyone interested in robotics.