What do you need to learn to write client-server/server applications?

Question

Java

Not so long ago I started learning java, I figured out the basics and understanding of OOP in general, I can write simple applications, I also have experience writing in php.
I want to start writing server applications, I have an idea for my project, I want to learn how to write client-server applications based on it.
That's the idea, I have a one-page site, there are fields (name, phone number, and other data), I want to send them to my server, process them in java and save them to a database or to a file.
The question is what is worth studying and in what direction to move? Is this how I understand java ee or the usual se? What should be written in this case sockets or servlets? What is needed to implement my simple idea?
Thanks in advance.