client server android - sending messages over the internet

I am able to make a client-Server application, and it works only when both are on the same wifi network. I want to make a client-Server application in which the server and client can be on different networks and have access to the Internet, in this case how do I make the application? Do I need to use GCM(google cloud messaging)? Please provide me the clear steps to follow to complete my desired app.