Where is a place to start in order to make a user registration system?

I would like to make a service in which someone would have to register (username/password) in order to use a service I am making. I am knowledgeable in Java/C++ however I have never done web development. I am not too sure where to start... how do I store usernames/passwords securely? Do I need to learn PHP/MySQL and get hosting for a server? It would be a web service though a mobile app I am making.

Thank you for your help and time.