A design brief is a paragraph or two explaining what your final solution will be able to do (NOT how it will do it!) as well as the limitations you have to work within. Your design brief should be able to be used as a rubric to evaluate how well your solution does its job.
A design brief should not mention the media you will use to solve the problem. That means you should not say "We will create a poster" or "We will create a website", just what the purpose of such media would be.
It should say things like, "We will create awareness in the Town of Grand Bank of a variety of mental health issues. All citizens will learn about mental health issues and will be provided with ways of finding more information."