Hard to know where to start answering your question!
If you want to combine learning more about photography with UW photography, it may well be worth getting a copy of Martin Edge's book, "The Underwater Photographer" (ISBN978-0-240-52164-0). At a modest 516 pages it takes you right through composition, camera settings, lighting - and the variations that UW needs. Even though it is, clearly, aimed at UW photographers, it does offer a very good guide to the basics of photography. I don't know if it's available in French but that might not matter to you.
I'd suggest the basics are understanding the effects and meaning of apertures, shutter speeds and ISO; then composition and how one impacts on the other; then lighting. There may be courses on offer in your community that could help you with these basic, but key, concepts.
But, as I say, if you can take a look at Martin Edge's book, I think that'd give you a great start.