One of the best pieces of advice I got before joining Hopin, was to take the necessary time I needed to be a "sponge" and let things soak in, before going straight into "solve-mode". Of course, that's easier said than done :)
When joining any startup as the first product marketer, you'll be getting requests from every angle from week 1(and sometimes before you even start!) - and that's especially true with PMM, because it is such a cross-functional role. This is what I've found to be helpful:
30 days: understand both the tangible and intangible working cultures of the company. How are decision made? Who are your main stakeholders? What is top of mind for each of them, and where is there overlap? Also use this time to develop your own "Product Marketing Charter" to do a bit of a roadshow and help others understand your mission and responsibilities (remember, PMM is still a fairly new concept for many!)
60 days: identify some quick win projects to start building your brand and help the company start to understand the value of PMM (develop sales one-pagers, put in a better release process, start tiering your feature releases, etc...) Also take this time to identify the tools and resources you need in order to do your job successfully.
90 days: by the 90 day mark at most startups, you're on OG! Embrace that feeling and remember that more likely than not, no one at the company knows more about product marketing than you do. Present your big ideas and long-term plans to your stakeholders, and see what resonates to help you prioritize where to execute first. This is also a great time to ask for any additional resources or headcount you need in order to do your job successfully.