Dont get too carried away with this for simple sites where a diagram and one to one explanations are best to convey your point. If you are building an APPLICATION, or hiring MULTIPLE developers then you want to have specs, prd's and more.
Might be valuable to write a website specification. The value of this, much like a business plan, is that it gets you to think about what you actually want. Also makes you think about other parts such as email, admin, cms.
http://www.scribd.com/doc/15430336/Website-Specification After that, a simple flowchart for the navigation, and some scribbles on headers, menu are whats best. If you are working with a Designer, they tend to do best when you can give them examples to sites you like, (with reasons) and sites you dont (with reasons). Look at your competitors also, for ideas on what to include in your site.
With a flow chart, and a separate word document per page you should be all set.
If you are building an application, you write a more detailed spec, or multiple specs for engineering, network, security. You also define guidelines for style, coding conventions, tools, etc. After that you can include a low level breakdown of the functions of your application.
Start looking for designers/developers in your area. This process is easier when done face to face assuming they are qualified. If you can't, then look on the web.
Find out how they go about working with clients and what they charge. They should be able to describe their process. Based on their feedback, you'll have a better understanding of what is going to be required of you. You'll also be able to compare and contrast different designers which will be a big benefit since this sounds like your first time working with them.