Can you teach me photography? That is like asking someone to teach him how to surf?
First thing is exposure. You establish your exposure using three settings: ISO (sensitivity of light), aperture (size of lens opening), shutter speed (how long the shutter stays open during a photo. Need to adjust these according to what you want to do, and they all interact with each other. For instance, higher ISO (say 1600) makes the camera sensitive to light, but at the cost of adding graininess to the image. Changing the aperture (say to 4.0) allows more light, but decreases the depth of field, the zone where the camera is focused. And making the shutter speed fast will freeze action (typically wanted for action photography), but will allow less light in.
Going auto-magic will set this all up for you, but to really use the camera, you need to understand exposure and let it work for you.
As for a housing for this camera, well, the D50 is the entry-level Nikon digital SLR. Street cost is between $600 and $800 (no lens). The metal housing talked about is the one made by
SPL Water Housings. They start at maybe $1500-1600. Need to determine which lens you will be shooting with, because each has its own lens port.
In my studies for buying a housing for myself, people have told me that it would be unwise to get a housing for my Canon XT (similar cam to yours)--that it would be a waste because the camera will not give you consistent results, and that resale will be difficult.
You need to determine how you want to shoot too. Do you want to go for the fisheye shot where you need to be withing five feet of the subject (think about doing that at Backdoor), or are you going to sit in the channel with a big lens (more expensive lens port)?
I could go on forever, but that's just a start. You can learn the basics, but the reality is that you need to just go out and shoot for yourself (27 years for me and counting). Also, each camera is different and you have to learn their idiosyncrasies.
If you have specific questions, just ask.