Writing a design brief

Firstly, I would like to mention that I was unsure where to post this. Mods, if you feel that this post should not have been made her, then please feel free to move.

So, writing a website design brief.

Well, I am about to pay someone to design me a website, but I want to basically layout all my ideas on paper so that he knows exactly what I require.

Has anyone ever wrote one of these before, and what exactly do I need to cover? I am a complete newb on this one, so any help is appreciated.