Well, experiment with different lines and find some you like. I usually respond to 1.e4 with e6, or the French Defense. Then, it all brancehs out from there.
Some openings have theory that goes past move 20. Should you learn that much?
No, stick to opening principles instead.
I am new to the serious study of chess. I just recently started putting some time into reading and studying the various aspects of the game. I am a bit confused on how you study openings. I watch many games and read about openings but am unsure how many moves does an opening consist of. Do I follow a pattern for 2, 3 moves? When would an opponents move make you want to change your opening moves? Sorry for such a complex question. I am just trying to wrap my mind around what an opening really is. THanks!