Assuming you have been assigned to manage a product that already exists, these are a few key things you should do:
Understand the product. This means understanding how it works, who uses it and why, what problem it solves, and what the roadmap looks like. Not all of this information will be available to you initially, but it's important to start gathering as much as you can.
Understand the competition. This includes understanding what other products are out there that solve the same problem, how they are different from your product, and what your product's unique selling points are.
Understand the market. This includes understanding who your target customer is, what their needs are, and how your product fits into the larger market landscape.
Understand your team. This includes understanding the skills and experience of the team you are working with, what their strengths and weaknesses are, and how you can best work with them to get the product built and out the door.
These are just a few of the things you should do as a new product manager. For more detailed information, check out this article: https://www.productivepm.com/blog/what-should-i-do