microcontrollers cannot control motors directly
You need something called an H-Bridge ( search google and this forum , there is tons about it).
There are H-Bridges in chips like the L298 that karmax mentioned, or you could make your own H bridge with transistors.
Also, for your robot , I advise you to use only two motors and a caster ( or two). This makes less frictio. But it depends on your application. What do you want this robot of yours to do?