Actually you should avoid splitting software into microservices by default until you really need it. It's easy to catch the moment - you will see that movement of some project part into microservice will help you to close many points and issues.
So:
1) Read classic books about monolith software.
2) Read decent books about microservices approach.
So: 1) Read classic books about monolith software. 2) Read decent books about microservices approach.