Hi A nia,
For any application to generate an ERD diagram (whether it be Visio or SQL Server Diagram), you need 2 things namely (as the meaning of ERD stands for), the entity and the relationship. The easy part is the entity, which is just that tables in your database. For the application to show the relationships between these entities, you should have foreign key relationships between your tables. Once you have these 2 things, Visio and SQL Server Diagram can already generate an ERD diagram for you.