To access the trailhead, go out Dave Wood Rd for about 25 miles, take a right on Divide Rd and drive a few more miles to the trailhead. There is a sign on the left that reads "Rim Trail." You can park here, then ride about 2-3 miles further up Dave Wood Road to the second marked "Rim Trail" on the left and start the trail here. Doing it this way, you'll ride the trail counter-clockwise.
The Rim Trail is a scenic doubletrack. There are a few rocky sections but it all is rideable for a beginner. It climbs up gradually to aspen groves and then to a scenic overlook with views of the San Juans. The trail then descends back to Dave Wood Road where you started.