Welcome to the board.
A search of the board will give you hours of reading.
To start you off, get close to your subject- a LOT closer than you might expect. The precieved distance and the actual (camera view) is quite a bit different. It's common for new shooters to not get nearly close enough and you end up with little small fish in a large background.
If you are using an internal flash (no strobe) getting close is even more important, your internal flash is good to maybe 3 ft MAX, anything past that is out of it's range. When using internal flash, I try to keep some type of solid background in the entire photo. If you have open water in the shot, you will tend to get a lot more back scatter.....looks like your shooting in a snow storm.
Get to know all the functions of your camera. Take some time and just shoot anything around the house, changing setting and reshooting until you have a good understanding of all the controls on your camera.
Be very careful when opening and closing your camera. Any small piece of trash on the O-ring will cause a leak. I never open my housing except when in my room, there is less chance of getting a grain of sand or hair on the O-ring in my room.
I would suggest that on at least the first few dives you spend some time working on your buoyancy skills without the camera. Without the camera, approch a subject to shooting range and practice framing the shot with your hands, hold the position for several seconds and then back away without touching anything or hitting the bottom, when you can easily do this, grab the camera.
Adding a camera adds a huge amount of task loading. Average skills are not good enough when you have a camera in your hands. Buoyancy, fin control and positional awareness need to instinctive.
