How do popular apps authenticate user requests from their mobile app to their server?

Question

Say I have an Android application that connects to a .Net API for receiving/setting data. The confusion that I have is regarding how to sign-up/login the user first time and

Answer 1

I was searching for exactly the same thing and found google way, something like peterpan said, but through Google APIs. Try this link and Google your way through it, I am starting also! I'll post new info while I`m at it!

http://developer.android.com/google/auth/http-auth.html