I believe this is when you should jump on Photoshop and make some Mock-Up Screenshots.
Before / instead of starting up this gigantomanium: Grab a piece of paper and a pencil and make a few mockups.
I wouldn't believe it myself, but I've been studying interaction design for some time now and there simply is no better tool to get your ideas out for the first time than pencil and paper (and maybe a ruler).
"But I can't draw." - Even better! It means you won't loose yourself in unnecessary details.
If you still feel the need to "improve" your work digitally, either scan it in and adjust values / contrast in Photoshop / Gimp or, if you wanna do anything more, use a tool that actually fits the task. It's called Photoshop, it's meant for Photomanipulation, and for almost any other task, it is not a good solution. If you wanna stay with Adobe-products, Fireworks, Illustrator or even InDesign are better suited to create mockups with. If you look outside Adobe there are many tools specifically designed for this task, several of them even online and offering a free trial (don't have a favorite here, so not going to post an example).
And, to not give the wrong impression that any of them would be the best way to do a mockup in this early stage: they're not, paper is. It's what the pros use, it's what education recommends and it's what experience tells us.
Hope you found this little pro-paper-rant helpful and maybe even enjoyed it a little (at least I enjoyed writing it),