Add security login to Web service in JAVA

I've got a web service server which takes a String and returns a string (an easy one).

Also, I've got a web service cliente in another computer who calls the method published in the server.

It works OK!

I've been told to add a security header, but I have no idea how to do it. I need to implement it in the server and also in the cliente. I've read I have to implement a handler but I don't know how.

Could you help me with this?