There are three primary ways that you create geodatabases in ArcGIS:
Design and create a new geodatabase (and subsequently define the schema and load datasets into the new geodatabase).
Copy and modify an existing geodatabase schema (and subsequently load datasets into the copied geodatabase).
Create a copy of both the schema and contents from an existing geodatabase.
Regardless of the method employed, all these alternative approaches are based on common design guidelines as well as considerations about key geodatabase properties.
Before proceeding with geodatabase creation, you should read about the following: