A common question is where to start now that I have my new rig. Here's some quick tips to get you started in the right direction...lots more can be found in the forum, too!
1 - put it all together on land and shoot away. Learn those buttons, see what the different settings will do, figure out the minimum focus distance (this is the number one problem I see - people remember to get close, then closer but forget that the camera does have a definite focus distance!) Shoot, shoot, shoot.
2 - Remember your shutter speed controls the colour of your water/background...slow (1/100) will give you lighter blues and fast (1/320) will give you darker blues. Really fast (1/1000) will give you very dark backgrounds - maybe even black. All values are approximate so you'll have to do a little experimenting.
f5 should be a good starting place for most subjects. If you are going to get very close and shoot tiny things, I would go to f8. I usually don't find a need to open beyond f 4.0 YMMV
3 - never take only one shot of a subject if you can at all help it! It's digital and hopefully you have a big card - if not, go shopping! Change angles and change your shutter speed (and/or aperture, but as a beginnner I'd concentrate on one thing for MOST of the shots)
4 - always download your efforts after each dive or diving day. Look at them and check out the settings so you can apply the corrections to the next dives. The magic of digital is that you can immediately correct your mistakes or fine tune images.
5 - start slow. Slow down your diving. No matter how slow you go, go slower. Also look for slow moving subjects - you'll have better luck with your images to start as you will have time to set up and review and reshoot. This doesn't mean don't shoot those fast moving fish, but does mean that the more practice you have on the corals, echinoderms, sponges etc you have the better your fish shots will become.
6 - get fast. If you are going to try to shoot moving targets, don't move your shutter below 1/250 or so. I've found that on some fish that are a bit energetic I need to move up to 1/320 or so to really freeze them. I rarely drop my shutter below 1/125 - this seems to be a safe speed to freeze many bits of action.
7 - get your hands on Jim Church's Essential Guide to Composition. A fabulous and timeless book that you will refer to again and again as you learn.
8 - practice more on land
9 - talk to your buddy before the dive. Explain that your priority is bringing back good photos and this means going slow, staying in one spot for a while etc. Make a plan. I often have my buddy do a spoke pattern from me...they can spot creatures, they are out of my shots, they get to explore a wider area. I get to concentrate on my shots.
10 - have fun!!!! Shoot til your fingers bleed