That's only the opening line to a full length novel....
Seriously, creating a relational database application is a significant undertaking that requires a lot of work.
As a general guideline, the first step is going to be to decide what data you want to track for that enterprise. Are you interested in Human Resources? In production of products? Sales? Shipping? Testing? Or something else?
Then you'll need to interview the potential users of that relational database application to learn what data points they need to account for. If it's about people, what demographics are relevant? If it's about production, what products does the enterprise make? And so on, collecting all of the required information is going to take diligence and good communication.
You'll also need to decide where and how to make that information collected in the relational database application available. That is, what kinds of reporting are required?
Security is another consideration. How sensitive is the information? Who can see it? Who can't? Are you subject to privacy laws?
Once you have a better understanding of these and other issues, you start by defining the data model on which your relational database application will be built. This is the single most important part of the actual development. Invest time learning how relational database applications must be designed to be efficient, reliable and secure.
Start, therefore, with an internet search on "Database Normalization" and really get a good understanding of that.