Do you have any advice for a junior PM who is a first product management hire?

Absolutely, welcome to the discipline, you're in for quite a ride and I'm super excited for you. 

1. Optimize for learning and honing your craft. There's a balance to have the confidence to make decisions without all the data and put stakes in the ground even you actually have no clue while balancing always opting to listen and learn and never actually being the expert on anything in the room. Confidant enough to leader while humble enough to learn.
2. Your career is a long ark. Think about when you started dating. You had no idea what you wanted to needed to be happy. Gaining experience in different areas, different teams and managers will help you put yourself in the right environment to excel
3. Finally use those same PM skills you build on shipping products on your career. What are your career success metric, what is your career roadmap, what hypothesis do you have about your career and how can you test them.

As I mentioned in another answer, lean on those around you; there is a wealth of knowledge to be amassed from stakeholders and peers that have likely interacted with the product that you've been brought in to support for some time. Those same team members likely led to you being brought on board to begin with, so learning as much as you can will set you up well for the future. 

If you can come up with a clear set of goals for yourself, it will be immensely helpful to gauage your progress. For example:

1. 30 - 60 - 90 Day Plan; there are plenty of online templates that can give you a baseline
2. Putting together a presentation of what you've learned for your leaders or peers after a certain number of months
3. Putting together a draft roadmap for the Product after certain number of months

The best thing you can do as a new PM is avoid putting undue pressure on yourself, or feeling that there is an expectation to be met. Putting in the work to understand what the product needs will generally land you in the right place every time.

Congratulations! Being the first PM at a company can be a really exciting and formative experience, in shaping the product vision and the product organization. Here's a few things I'd suggest:

• It's all about the user - user needs, user problems, user stories! Spend a lot of time learning about your customers, their needs and what they're looking for from your product. 
• Make sure you socialize what product management is, what your key responsibilities are, and how cross-functional partners should work with you. A "PM" can mean different things to different people, so make sure you tailor this to what your company needs and then communicate it out.
• Set up your product processes early. For instance - start simple with a product launch calendar, some task management process (e.g., an agile process in JIRA or Asana), etc.

If you are a junior PM who is the first product management hire, it would be fair to assume you are working at an early-stage startup where likely the founder is acting as the product leader. You were hired to support the founder and your focus will be more on execution versus defining strategy and vision. This would be a great opportunity to learn the ropes and over time if you can showcase strategic thinking there may be additional opportunities that can open up.

Your focus should be learning as much as you can by taking up whatever you can even if it is not in your job description. Take time to understand the context in which you will be working, get a clear understanding of goals and priorities, be proactive versus reactive, communicate effectively, and lastly don't be afraid to seek mentorship and help. In your first month, focus on the following -

• Get to know the team and the business - Take the time to learn about the team you will be working with, as well as the business and its goals. This will help you understand the context in which you will be working and will inform the decisions you make as a product manager.
• Understand the product - Make sure you have a deep understanding of the product, including its features, benefits, and target market. This will help you make informed decisions and communicate effectively with the team and stakeholders.
• Understand product goals and priorities - Get a clear understanding of goals and priorities for the product, and communicate them to the team. This will help everyone understand what needs to be done and why, and will ensure that everyone is working towards the same goals.

Assuming this is an early stage company, the priority should be learning the basics of the business and building relationships with as many colleagues as possible. Use those learnings to construct a map of current products, problems and opportunities. Considering you are the first PM, any foundational product-related frameworks you put in place will be welcomed by most of your colleagues, so don't be afraid to suggest new ways of working. I'd also suggest reading some of the popular PM books which contain some really helpful tips and tactics: The Lean Product Playbook, Inspired, Product Management's Sacred Seven, The Product Book - How to become a great product manager. Good luck!