How do I read and write data from cloud server using C#

I want to make a log in system for my android application. For that I need to store user name and password to Cloud Server and retrieve that data during login. If user name and password have match case then individual can able to access my application.

So I want to know what are the required things to know or please provide me sufficient links to learn how to data can be read and write to cloud server using c#.