Congratulations, you are really going to enjoy this! The previous posters probably know more than I, but since I too, am a novice, I will add my bit. I sometimes shoot in Program mode and note which settings the camera chooses and then change to Manual and experiment. It gives me a baseline when I do not know where to start (most of the time). Also, I found it helpful to put some little sticker cheat-sheets inside my housing. For example, the shutter speeds and f-stops recommended by the above posters can be written on the sticky-side of a label and placed, facing out, on the inside of your housing. (pick an unobtrusive area) This is helpful for me as I get very "task loaded" down there and it is helpful to refer to these settings. I need one that says "hey, stupid, CHECK YOUR AIR, and your BUDDY!"
I get some decent shots, especially with the wide-angle, shooting in Program mode, especially when there is a lot of action and I cannot take the time to change variables.
Have a great time and be on alert to your diving, because, from now on it is a different world down there!