First decision you will have to make is what processor to use. Some of this decision will depend on your expertese, some on what you want the robot to do. You might even use several processors on one robot, maybe a PC acting as the brain, and then sending a message to a PIC which acts as a motor controller. Some of the more common processors to use are the MicroChip PIC, eg, 16F877 which has a limited memory capacity, runs at 20Mhz, but has a few nice features built into it. Easier to program is the Stamp range which can be done in BASIC. Getting a little more in depth is the AVR range of processors.