This Google Document comprises of the most common elements required in correctly briefing in a digital marketing agency. In total, it is about 4 pages long so it is not super short but not too long either. You are welcome to use this template as a basis for creating your own.
Providing an accurate timeline is important to ensure expectations are understood from the start. The same goes for your budget. Being aligned on budget is one of the most important parts of working with an agency
Providing context to your previously run marketing activity and the maturity of your marketing operations is really helpful for an agency to get some context. This helps them cover new ground and not going over things you have previously done or tried.