I took a underwater photography class where we learned how to change the settings on the camera for point and shoot shots. I'm sure the links that people provided explain what settings to pick, don't just use your auto settings since those are meant for land shots.
I did the class portion and was supposed to do a pool session also but we had some serious blizzards last year and my pool session was canceled. I went on my trip to Cozumel and used my digital with external strobe for the first time and my shots turned out pretty good. When I got back I finished the pool session and it definitely helps. A couple that works at our LDS created an artificial reef with fabric and stuffed animals and it was fun practice. My instructor also reiterated what another poster said, practice your focus on land by pressing the button half way down. I haven't done a lot of macro stuff but I've heard it's hard to switch between the two when your underwater so I would just pick one for the dive.