database for my app

[Abdelillah ARICHE]

I want to create an application, I will save the data of my users in my desktop, and I will send them some notifications every time.
I don't know any database management system, So my question here :
what is the best database management system I have to learn and that will stay useful for a long time in the future ? (I do not want to spend my time learning things that will disparate in the few years coming).
what is the best tools (protocols) to interact with my application's users ?
Need your help please