well if you are asking this question, you probably don't have much microcontroller experience. A good beginner choice would be one that can be programmed in pBASIC such as the STAMP microcontrollers from parallax.
http://parallax.com . All you need is a demo board, and a STAMP.
Alternatively, you can use a PIC. To program it in C you will need a compiler such as C18. The version that comes in the FIRST kit will only work on the specific PIC that is used inside the robot controller. If you download the demo version off the microchip website, it will work on all PICs but expires after some amount of months. If you use a PIC, you will need a PIC programmer, and some sort of circuit to house the thing. It will need at the very least a battery, a voltage regulator, and and oscillator of some kind to provide the clock signal. PICs are far more powerful but have a steeper learning curve.
If i were you, i would use a STAMP. A kit like
http://www.parallax.com/detail.asp?product_id=27206 would have everything you need to get started and learn the how to use stamps. The manual that comes with it even has examples of how to control servos. Its kinda pricey but worth it.