If you want a broad overview of how RDBMS work under the hood, "Designing Data Intensive Applications", would be a good conceptual start.

This could be followed by database internals books like "Expert Oracle Database Architecture" and "Pro SQL Server Internals", irrespective of whether you use these databases or not, you will learn a ton of stuff about database internals and system design.

If you are interested in database schema design etc, then look up "The Database Model Resource Book" Volumes 1,2,3. None of the above are quick reads but they are solid foundations for becoming a well rounded DBMS professional.

If you want to learn the basics of creating a database schema (i.e. creating tables, defining relationships, normalization etc), this Lynda (now LinkedIn) course I think is the best for beginners. It does require a subscription to Lynda/LinkedIn to watch which may not be practical.

Here is a video excerpt from the course to give you a flavour of the instructional style:

https://www.youtube.com/watch?v=V5DyvUfsboA

Are you talking about structure of DBMS? Or how to use them? What are the exact topics you have to learn and are struggling with?

I mean topics like these

https://www.youtube.com/watch?v=5fs1ldO6B5c&list=PLxCzCOWd7a...