I would find it pretty hard to give generic advice on this question. But you can learn a lot fairly quickly from on line resources. Here are a few good places to start in no particular order:
https://www.opticaloceansales.com/handbooks-basics-of-better-uw-photos-en.html
http://www.theunderwaterphotographer.com/
http://www.uwphotographyguide.com/
http://www.underwaterphotography.com/
http://www.divephotoguide.com/
Like others often recommend, I found that Martin Edge's book "The Underwater Photographer" is an excellent beginner's course to get up to speed.
Looking at other people's work and trying to understand how they made an image is a very good way to learn. There's Wetpixel of course, sites of the pros, competition results, the Nat Geo photo if the day UW category, lots of Flickr groups, etc
The trick is remembering all that you need once under water!
Finally, if you are a new diver, the most important advice I can think of is to get good at diving before even thinking about taking the camera along. Buoyancy control, safety, and making sure you don't run out of air come first and need to be close to automatic. You don't want any chance of photography distracting you from staying alive!