Don't over think it. Keep the creative process simple. I have found that the "learning curve" for most tools/software can hinder the creative and planning process.
1. Outline your site idea on paper first. Go to Wal-Mart and buy a flip chart (I like the Post-it kind b/c you stick them on the office wall)
Once you are satisfied with what you have planned out on paper, find a graphic designer who can help you create some "mock ups." Design concepts and screen shots will really bring your idea to life.
2. Another suggestion would be to go to Home Depot and buy one of those 6 foot sections of white board. If you think you will be erasing a lot, this might be better for you. Home Depot sells this much cheaper than Wal-Mart and has bigger sizes (which it sounds like you'll need
Good luck!
-Brandon