IMO, you don't need a lock for short trips into the store, just keep it in sight, or bring it in with you. I don't take a lock with me on my training rides. If you're going to actually leave it somewhere for a little while (say, ride it to school) I recommend a cable lock plus a U-lock. They require different tools to defeat, and make it pretty obvious during the day. These aren't things I would want to carry with me on a training ride though, just short commuting type trips.

Lock the U-lock around the frame and front wheel. Lock the cable through the frame, rear wheel, and front wheel.