The Web Designer Depot have put together a great article on how to create a good design brief.
As a designer, you need to capture what the company does - their niche market and USPs. Importantly, you need to capture what they want out of the piece of design work.
Budget and time constraints - obviously. This reminds me of the web triangle - Good Fast Cheap - pick two.
Katherine Writes for the image)
Getting all of this information out of the client (including their likes, dislikes, colour preferences etc etc) can take the form of a questionnaire, an interview, or providing the client with a guide on how to write a brief (if they've not done it before).