XML : Displaying variables as text in Android Studio

I'm new to mobile development and I am having trouble displaying variables as text on to the screen.

  public class wifiinfo() {      public string getmac = getBSSID();  }    

I have this in my main (blank) activity but I have no clue as to how to code this in with XML. The Android Developer tutorials haven't really helped me. Do I have to index the string in a database, perhaps? Any help would be greatly appreciated. Thanks in advanced.