In the Submit or Login button on click event is where you put your code to check for the correct username and pw. Most likely this data will come from your SQL database or a flat ASCII text file.
Once you determine they have logged in correctly, then you StartActivity("TabControl") or what ever the next (Activity) screen is you want to take them to.
If they don't login correctly, then you stay on the same login screen and display an error msg; incorrect username or bad password, etc. A dialog box would work nicely here for that.
