[Alan Anderson]

You'll need some sort of feedback to tell you what position the motor is in. Usually that'll be a potentiometer. Then you'll need to write a program to compare the measured position to the desired position, and drive the motor in the proper direction at the appropriate speed to get it to where you want it to go.

Search the forums for "PID Control" and you'll find several discussions of the topic.

Edit: Search the forums for "PID Control" and you'll also find out what it is.