Who "decides" which opening to use?

I'm a beginner, investigating the idea of openings, and can't work something out. In the first part of the top answer on to this question on the Chess stack exchange  the answerer "recommend[s] the Sicilian defense against 1.e4, and the King's Indian defense against everything else".

Regardless of whether that's sound advice or not, does this mean that it is ultimately black, with his response to whites initial move(s?), who determines which opening is played?

Related questions are, if this is the case, how can white prepare (strategize) for a game ahead of time? 

The question at the bottom of all this is "where should I begin my relationship with openings?" Advice seems to vary from "you must know your openings" to "don't worry about openings and focus on (some other aspect)". In either case, I'd really like to have this mild confusion about how openings "work" cleared up and would appreciate any input.

You should start your relationship by looking at opening principles first.

Some openings are determined by the white pieces. Some are determined by the black pieces.

Also, within those systems you can have moves by either side that define a variation (A system within a system).

You are asking a very good question.

For example:

1.e4, e5 2.Nf3, Nc6 3.Bb5 is the Ruy Lopez.

Instead of 3.Bb5, white may play 3.Bc4 which is the Italian Game.

If black continues with 3...Bc5, we have the Guioco Piano variation of the Italian Game. White defined the opening; black defined the variation.

If white then continues with 4.b4, we have the Evans Gambit variation of the Guioco Piano within the Italian Game.

Another example is: 1.e4, c5 - Sicilian. Black defined the opening.

It might continue 2.Nf3, d6 3.d4, cd4 4.Nd4, Nf6 5.Nc3, a6. the move a6 defines the Najdorf. Black has other moves besides a6 and each of them have a name.

g6 - dragon

Nc6 - classical

e6 - Scheveningen

If after 5...a6 it might continue with 6.Bc4. This is called the Sozin.

That may seem like a lot of info, but it's not that hard to wrap your mind around it when you only study games that open with 1.e4. If thats the case, then....

e6 - french

e5 - can be a lot of things depending on what happens

d6 - Pirc

d5 - Scandinavian / Center Counter Defence

c6 - Caro Kann

c5 - Sicilian

Nf6 - Alekhines

There's others. You just need to look em up.

I think it goes like this

Opening (1st branching point)- Black

2nd Branch - White

3rd Branch- Black

4th  Branch - White.

An example of this would be:

Have you noticed that your comments are usually pointless and unhelpful? You usually post diagrams with no written explanation whatsoever, and most of those diagrams are completely irrelevant.

Have you ever noticed that you sometimes act like a moron. If you want to insult people this is not the place. Let me guess, you are not insulting anyone, and I'm not insulting anyone either. Who's irrelevant? Who you've been calling a moron?

eagiraud, don't waste your time. Leklerk (or another alias) does it on purpose.

It reminds these South park's episodes on trolls.

Pestebalcanica, is that you?

What the [really don't need that, even obfuscated -- MOD]  do you care?

Dont worry about the Irish dude my friend, just listen to your heart.

Here, take this blunt, and let peace take over your mind:

[uncalled for -- MOD]

Lol. Now that I have teased the Irish dude I can report him so he gets banned for being abusive.

Right. They are teasing a lot. You are not the only one.

We are not teasing, we dont like to see your non-sense, it just takes space in the thread an doesnt apport anything for the conversation.

I have just reported BronsteinPawn. Just wondering.

Back to the topic and the OP. I believe since white begins EVERY game white decides. Unless of course black is a "better" player and knows a proper/unsettling response.