Whenever you are starting a new role, it's critical to understand what's important to your manager and what the objectives are for your new organization so you can align yourself well to them. Every company has a different onboarding plan, and for PMMs I think it's critical to get the lay of the land through meet and greets with the people you'll work with to hear first hand what is on their mind, so you can start to understand how you will work together. I also work with my manager to define what I can deliver as soon as possible to show impact to the organization.
This is not PMM specific, but there is an app that I use called the "The First 90 days" which I use to help me think through different elements of my new role. The information is useful enough that you can adapt it to a PMM role.