An administrator can set his/her menu view to everyone.
This feature is available to CRM+, Project+, Standard and Professional plans users only.
First of all, you need to configure the main menu and add any custom items you need.
Read more in the article - Add items to the main menu.
Then you need to click Configure menu > Set my menu for everyone.
Specify users your menu will be set for (new users or all users) and click Save.