Since you are talking PG_ADMIN, you must be interested in the PostgreSQL DBMS. Are you interesting in learning more about using, developing against, or administering PostgreSQL? If you are interested in learning more about PostgreSQL, there are numerous good offerings you can find by searching the internet. If you are interested in learning more about enterprise geodatabases in PostgreSQL, I would start by looking/taking some Esri training on geodatabases first. Once you understand how geodatabases work, in general, you can start to learn more about the nuances of implementing geodatabases in PostgreSQL.