I can field the one about positions:
That depends on your relative heights and the structure of your shower. Putting her against the wall, like you're going to frisk her, and entering her from the back can be cool. So can just lifting her up so that her legs are around you.
What you'll find is that sex in the shower is one of those things where the reality doesn't measure up to the fantasy. Generally, someone is getting sprayed in the face, which is unpleasant if it continues while you're trying to breathe. Getting started in the shower and winding up in the bed or on the floor can be an incredible experience, though. Be sure and shampoo her hair for her. Girls like feeling taken care of like that.