I think you need to spend a little time clicking through opening moves in a database. Try googling "free chess database" if you don't have one. Black is free to respond to white's move any way he wants. Yes titled players spend a lot of time on opening theory since they've mastered the rest of the game, but for the rest of us adhering to opening principles is much more important. I think you could use a nice gentle structured approach to learning opening principles and seeing them in action. Try these free courses; they were designed just for players like you.
http://www.chess.com/blog/webmaster/free-chess-mentor-courses
If white chooses ( X opening, defence or gambit ) does black choose their own game? Do both have to figure out which opening it is and play those moves?