We've created some getting started guides for using the gradebook, depending on whether you weight your grades or just use straight points. While they were written for 3.1, you'll find that the general layout and function are not very different in Moodle 2.x on.
Other aspects of using the gradebook:
Virtual Academy of Teaching and Learning
E-mail: vatl@ysu.am
Phone: (+374 10) 57 06 77