Ah, so you're starting from scratch with learning how to build? You can get started with this learning process even before Seven releases the new scripts; you can always retrofit any fish you build to work with the scripts, after all.
I alread knew Photoshop but in terms of SL building, here's what helped me...
I visited the Ivory Tower Library of Primitives as a starting point. They have a great self-guided tour that shows you all the different things you can do with prims and what shapes you can make.
http://slurl.com/secondlife/Natoma/209/164/27After that, you'll start analyzing everything you see in SL with an eye to how it was assembled.

You can study some existing fish to get a sense of how they were made, for example. The fins are usually skinny flexible rectangles with a clear pixel on all the edges, and a fin texture on the two main sides. Note that when working with flexi shapes, the location of the flexi's "root" is really important. They'll explain all that at Ivory Tower.
You could also take some classes if that'd help -- I took a few at Rockcliffe University. See their Schedule Board.
http://slurl.com/secondlife/Rockcliffe%20I/100/99/24As for sculpties, I use Rokuro Pro which works kind of like a lathe -- you know, like in woodworking, to make fancy chair legs and stuff? Or like a potter's wheel. Basically you define the outside shape you want in profile, and it makes the sculpt for you. It's great for making organic egg-shapes and fish body shapes and such.
http://kanae.net/secondlife/rokuro_pro.html