In my experience the Product Owner is the most underestimated role in Scrum. I think it’s also the root cause of many problematic Scrum implementations.
Every Scrum implementation should start with the Product Owner. You should be very clear about the responsibility of the Product Owner:
This means that a product owner is much more than a user story writer. Most Product Owners I’ve met so far are requirements analysts with a new title. They are not empowered to sort the backlog, they are not empowered to learn from feedback and improve user stories.
If you are about to start with Scrum, start with the product owner. This will be hard enough, as many companies do not have a role or person responsible for the success of their products.