[eluser]webscriptz.be[/eluser]
You know one way to break things down easily is:
take a sheet of paper and not everything you think you need for the system, then search only for the other systems and see what you miss, try demos of the systems and note their mistakes so that you don't make them.
When all of that is finished, note the controllers and models you need, and try linking them together.
Make your database and begin coding the simplest thing of the website, you need to start somewhere so...
And last but not least use a very very simple template like the CI default to test everything before incorporate it in a design, if not you'll end up compromising with the design and the things "this is nice" and "this doesn't go so well"