One of the first thing need to be defined, are entities. Entities should be defined like they really exist in business process. Latter they will contain real data in application.
Examples for entities are Product, Product category, Customer, Orders, Orders items.
Every entity contain attributes, and every attribute have attribute type. With attribute type it is possible to connect entities.
For example, entity Product have attribute Product category with type Selection (one selection) from another entity Product category. Entity Product category have one attribute Product category name with type Text (one line).
After saving entities, database is generated with all tables and connections.
For more details see How to define entities
In current version, if you change name or type of existing attribute, after saving all data will be deleted from that attribute.