As a young developer I stayed away from traditional relational databases because I thought they were boring and restrictive. I quickly fell in love with them. I realized they painlessly provided all the things I was already doing with data and they were doing it much faster and more reliably.
I hope you have the same kind of experience and enjoy Postgres!
As far as actual advice...
1. The key concept of an RDBMS is that each table typically specifies relationships to other tables. So in order to query data you will typically be joining multiple tables. Sometimes new DB programmers get a lot of enlightenment from the simple Venn diagrams that illustrate the basic types of JOINs: https://www.google.com/search?q=inner+join+diagram ...IMHO, once you get this (fairly simple) paradigm the rest is easy.
2. Database constraints are your friends. At a minimum you specify the types of the columns, obviously, but you can go much farther - specifying NOT NULL constraints, foreign keys, and more complex check conditions. Do as much of this as feasible at the database level. Otherwise it is something you need to code and enforce at the application level, where it will generally be less performant and more prone to coder error. Additionally, any constraints not enforced at the application level will need to be implemented across multiple applications if more than one app uses the database.
The second one above is an example of something that sounds boring and restrictive but really frees you up to do other things as a developer. About a decade ago, the "trend" was to treat RDBMSs as "dumb" storage and do all that stuff at the application level, in the name of being database-agnostic. Opinions remain divided, but I think that was an objectively bad trend. For one thing, folks noticed they hardly ever needed to suddenly switch databases, but there are other reasons as well.