Hey Hunter,
You may be rushing things a little, Yoobee might not be ready for that yet. What you said makes sense though, but I'm sure you mean't to put the chord not the key in your post.

It's the root note that decides what the chord is, and that's the first note in it's major scale. The other notes of the chord are the 3, and the 5. Just to give an example, take the G chord for instance.

G A B C D
1 2 3 4 5

Every note in a basic G chord is G B D, anywhere on the fretboard that you can put these notes together is a G chord. This is called a triad, and it's a good idea to learn the triad for every chord.

It's also a good idea to learn the chord progressions in the keys. Basic chord progressions are the 1, 4, 5 chord in each key. Here are a few examples of this.

1 2 3 4 5
G A B C D
D E F G A
A B C D E

The 1 is the key you're in, and the 4, and the 5 are the other two chords in a 3 chord progression in that key.

If you're reading sheet music, you look at the key signature. The key depends on how many # (sharps) or b (flats) you see in between the treble clef, and the time signature. The G major scale has an F# in it, so if there's a # sign on the top line of the staff (F) the key is G. I think key of F is the only key with a b (flat) in it, and the note is a b flat too.

Let me know if this helps Yoobee, and if you have any other questions. I don't know where you are in our courses, but I do know this info is part of the Core learning system.