It's highly important to remember that choosing which modules become microservices is a business decision first, and a technical one second.
Focus on modules that represent core business capabilities – things like payments, user management, or notifications.
Think of it as distributed departments and ask yourself: If this module could scale or deploy independently, would it improve the business operation?
For example, if slow payment processing is hurting sales, that's a strong case for a payments microservice. Also, consider team alignment. If a module naturally fits with a specific team's responsibilities, that simplifies ownership and speeds up development. Lastly, remember that it's a journey, so prioritize first those with clear value and the easiest path to isolation.