How do you know which opening to play?

As a new(ish) player, I've just started to study the game and have encountered what seems to be an ambiguity. I keep reading that I should pick an opening and play the heck out of it for a year to really absorb it. That, I understand. What I'm less clear on is how I get my opponents to go along with that.

Say I'm learning the Benko Gambit and want to play it as black. How do I know that white is going to play 2. c4 (which seems critical) after I play 1 .. Nf6? Isn't there like a ton of other things white could do that aren't c4?

I guess I'm wondering if both sides aren't just reacting to what the other player is doing while trying to follow sound opening principals and the opening is the one that most closely matches what they played rather than being something that one of the players decided on?

I apologize if this has been asked before; I didn't find a similar post via the search.

Thanks!

You dont get your opponent to go along with what openings you play.  You need to understand the pawn structure, piece placement, and endgame ideas of each opening.  For now...I wouldnt get all caught up in openings per se.  I would concentrate more on Opening principles for now.

Opening Principles:

1. Control the center squares – d4-e4-d5-e5

2. Develop your minor pieces toward the center – piece activity is the key

3. Castle

4. Connect your rooks

Tactics...tactics...tactics...

Pre Move Checklist:

1. Make sure all your pieces are safe. 

2. Look for forcing move: Checks, captures, threats. You want to look at ALL forcing moves (even the bad ones) this will force you look at, and see the entire board. 

3. If there are no forcing moves, you then want to remove any of your opponent’s pieces from your side of the board. 

4. If your opponent doesn’t have any of his pieces on your side of the board, then you want to improve the position of your least active piece. 

5. After each move by your opponent, ask yourself: "What is my opponent trying to do?"

Here are some suggestions from me:

-Queen's Gambit

-Cozio Defense

-Nimzo-Indian Defense

Learn the basic first few moves of the openings and the early ideas. Then start playing them, and try to come up with the best moves in your games. Remember to control the center and develop your pieces. Watch for tactics and threats, and improve your pieces. Learn the theory and ideas as you go along. For my first year of chess I learnt the Queen's Gambit as white, and bit by bit I went through black's main options, did my online research and built my repertoire. If you see a move your opponent played that doesn't really follow what you know so far, or you know about the variation but haven't booked up on it, see if you can do research to find out about that particular variation and know what to do for next time.

"... there will come a time, whether on move two or move twenty, when your knowledge of theory runs out and you have to decide what to do on your own. ... sometimes you will leave theory first, sometimes your opponent. Nothing will stop this happening. It happens in every well-contested GM game at some point, usually a very significant point. This is a part of the game: an important part, ..." - IM John Cox (2006)

As one improves, one's games gradually tend to involve longer stretches that go according to book.

"... Review each of your games, identifying opening (and other) mistakes with the goal of not repeatedly making the same mistake. ... It is especially critical not to continually fall into opening traps – or even lines that result in difficult positions ..." - NM Dan Heisman (2007)https://web.archive.org/web/20140627062646/http://www.chesscafe.com/text/heisman81.pdf

2. c4 isn't critical. There are indeed many other good moves. So U should prepare replies to the knight games 2. Nf3 and 2. Nc3, London system with 2. Bf4, Trompowsky attack 2. Bg5 etc. 

If you're new to chess, then start by playing "960 chess". You can't fall into any opening traps that your opponent has memorized.