When starting with robotics a number of problems present themselves. Firstly How good are you at electronics, then how good are you at electro mechanics, then how good are you as a programmer.

It is wise to not try and battle on too many fronts at once.

This is why my first robots were build using the Parallax standard stamp board from Milford Instruments. This allowed me to concentrate on learning about how to build a drive train how to build sensors and all the electromechanical tricks and components that are necessary to make the programming work.
The actual board I used is no longer marketed by Milford but these two alternatives are available. Be aware that this is a stamp 1 board and is quite limited in its programming ability.
This board contains a stamp 2, which will be a stamp 2 of your choice the basic board will come with no processor. There a a number of stamps each with different capabilities.
The chip on the prototype board is an L293D motor control chip, this makes it easy for your processor to control a pair of motors in forward and reverse, you do need 4 pins on the microcontroller (Brain) to make it work. A bit like tank / bulldozer steering the left lever is either forwards or backwards and the track on that side goes the same way, and the same on the right. One pin for each direction. If you say back and forward on the same side the chip doesn't short out it just does nothing.
Cheap sensors as the brass wire bends it touches the side of the tube and makes a circuit. One wire goes into a pin on the chip and the software monitors that pin waiting for the signal then it reacts.
LCD screen at the back is used to send messages from the software to help with debugging.