title: How to make a fragment open by default in Android? How to sign button which is clicked?
body: <p>I'm making football application and it has 4 buttons on the top side of the screen. Every button will open different fragment. Now when I open the app by default it's showing 4 buttons and empty field under them where will be shown content of fragments. What should I do so that one of fragments will be opened by default(as MainActivity but it should stay fragment)? Further, how can I sign(note, mark) the choosen button so that user will see which button is selected now? 
  Thanks, if something is unclear tell me and I'll explain further</p>