Beginners in photography can (understandably) get confused by the fact that big numbers mean small apertures, or that shooting 'wide open' implies a small depth-of-focus. I liked this mnemonic from Chris Orwig on a recent This Week in Photo podcast:
Imagine you're shooting a line of people. If you want one person in focus, use f/1. If you want all 16 in focus, use f/16.
Nice and simple.