After installation I would recommend reading the Manjaro Wiki, especially the section called “Customization and Configuration”. It contains a lot of useful information:
https://wiki.manjaro.org/index.php/Main_Page#Customization_and_Configuration
Another good read is the “General recommendations” from the Arch Wiki:
https://wiki.archlinux.org/index.php/General_recommendations
A lot of the things mentioned in this page are valid for Manjaro as well. Overall, the Arch Wiki is the first place to search for information regarding Arch or Arch-based systems.