If you're staying in SF and just want to ride it for a bit to see what it's like, I'd recommend starting at one of the downtown stations (Civic Center, Powell, Montgomery or Embarcadero) and take a train to the East Bay (Richmond, Pittsburg/Bay Point, Dublin or Fremont lines) through the transbay tube. Get out at West Oakland, turn around and take a train back. As others noted you can save some money by exiting one station away from your starting point.