Get into a rhythm by understanding the business/company goals. Then understand how the tech works (look at the customer/user interface first and then make sure you understand how the system behind the UI works). Begin collecting data in a transparent way and share your learnings with your stakeholders and leadership.
Conduct user research (both qualitative and quantitative) to help illustrate what product areas (epics/themes) you want to focus on and how these connect back to the business/company goals.
First PM in a company! I have not done it, nor have anyone in close network to have a good understanding. My guess is that they have to establish right roles/responsibilities on what to carve out from the CEO. Perhaps focused on scaling up product for next million users (or take on next set of enteprise clients), or execution focsed. Do take this with a grain of salt as I am guessing based on when should a CEO hire their first PM.
Interestingly enough I see two trends in the types of KPIs product teams miss.
1) Aligning with the larger's organization or business goals - Ensuring that your product roadmap is actually impacting the success metrics (OKRs, KPIs) of the business itself is critical to knowing if you are investing in and prioritizing the right work.
2) Capturing "technical or engineering" metrics - Any work that your team spends time on should be impacting some metric. Even metrics that are technical (the most common one being latency) should be captured, reported on, and measured over time.
Product teams in my opinion consist of product, engineering, and design (at a minimum). With that said, product KPIs should always be shared with engineering since what they are building essentially impacts the KPIs of the product in question. All the work that product teams do should always build up back to the overall company/business unit objectives (even if those metrics are more technical).