1. The first thing about keys is you really need to know just one to start with, Am/C. All the white keys, no sharps or flats. Makes it easy to find home base, which is (for historical reasons I will elaborate on later) Middle C.
2. Next, I call it Am/C because both Am and C keys use the same notes in the same order, except your starting place, the “tonic” is either A or C depending.
3. Learning the Am/C scale on the guitar is pretty much essential if you want to learn to play well and understand (pesky brain) how music fits together. Here is a PDF of the Am/C scale and chord for reference and practice: C-Am_chords_scale.pdf
4. The key to finding the key of any given piece of music is finding the One (the first or tonic chord/note in the scale and chord progression), the Four (the fourth chord/note), and the Five (the fifth chord/note). 80% of all western music is structured around these three chords — the 1, 4 & 5 — or around some modification or avoidance of them (really, see: https://www.guitartricks.com/forum/showthread.php?t=37751)
5. We’ll talk more about minor chords and keys later.
Example:
Am
1 – Am (tonic)
2 – Bm-5
3 – C
4 – Dm
5 – Em
6 – F
7 – G
1 – Am (octave)
C (Major)
1 – C – tonic
2 – Dm
3 – Em
4 – F
5 – G
6 – Am
7 – Bm-5
1 – C (octave)
Other common guitar keys:
G
1 – G
4 – C
5 – D
E
1 – E
4 – A
5 – B
A
1 – A
4 – D
5 – E
D
1 – D
4 – G
5 – A