The brief should answer four things
Who is it for?
Describe the person and the moment when the app becomes useful.
What happens first?
Keep the first version small enough that a real person could try it.
What does it look like?
Give the visual system and screen flow enough detail to guide a prototype.
How will it travel?
List the search, share and launch paths that can bring the first users.
Free starting point
Use Palm's free launch brief to test the shape. The Mac app is for founders who want that thinking connected and reusable as the product changes.