So you have a great idea for an iPhone app or a website but not sure where to start? Use a prototyping tool to layout the wireframe.
Wireframe = "is a visual guide that represents the skeletal framework of a website... The wireframe depicts the page layout or arrangement of the website’s content, including interface elements and navigational systems, and how they work together. The wireframe usually lacks typographic style, color, or graphics, since the main focus lies in functionality, behavior, and priority of content... Wireframes can be pencil drawings or sketches on a whiteboard, or they can be produced by means of a broad array of free or commercial software applications."
This is what one looks like:
You can also do wireframes for iPhone applications. When you're ready you can actually make the wireframe interactive by adding in the ability for testers to click through like they would with a live website or iPhone application.
1. LucidChart (doesn't have interactivity but an easy way to diagram something),
2. JustinMind (it is interactive and super simple to create and share, but it has some limitations like no scrolling when doing iPhone app mocks), and
3. Axure (this has more bells and whistles but takes more time to learn).
Don't worry about the price. They all have free trials. Try them out before you buy to get a sense of what's possible.