There are tons of templates (both blank and in stock layouts for all studios), logos, fonts, and finished custom covers at Dagon's Den. They also have a paid subscription resource area with high-res poster art, stills, credit blocks, etc.

As for Photoshop knowledge and/or skill, it's something you just pick up through books, online tutorials, and way more practice and noodling around than I've put in thus far. The basics are fairly easy to get down, it's the little tricks to digitize imagination that take time and patience to learn.