ok so i am new. been playing since the day after christmas. I play most of all my music in standard tuning and have also stated using a capo. but everyone I play with always ask me what key i am playing a song in. I am unfortunatly teaching myself and i have been having pleasing results, but i have no idea how to answer their question about what key i am playing in. Please give me some guidance. thanks.
Basically, when you play a song you will be following a certain chord progression. Depending on which chords or notes you are playing it will be in one key or another. For example: If you are playing a chord progression based on C/Am/F/G your song will be in the key of C major. If you teach yourself you will need to learn some basic musical theory if you intend to write music.

Hope it helped :s
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Cmajor scale
C - D - E - F - G - A- B
A - B - C - D - E - F - G

if you play chords and they all consist out of these notes you are in Cmajor or Aminor.

So if you play Em - D - A you are in the key of Dmajor

Everything transposed a note from Cmajor
D - E - F# - G - A - B - C#

And that`s the way the cookie rolls. But learn some theory. You really need it...
You can also take the aural (or the analytical) approach to finding out the key. The aural method (listening to where the song wants to resolve/does resolve) and the analytical method (looking at how the chords function and lead into the tonic, which requires a deeper understanding of harmony and theory) are much more effective when there are notes that are out of key, as just looking at the notes in the song won't yield any result.

For example, if you have a song that goes C F G Ab Bb C, looking at the notes won't help you at all, as there are two chords that are out of key.
C - C E G
F - F A C
G - G B D
Ab - Ab C Eb
Bb - Bb D F

You'll see that the notes in the song are C D Eb E F G Ab A Bb B. That's more than seven notes, and it doesn't fit any specific key. But if you listen to the chord progression, you'll hear that it is resolved on the C. If you understand the theory behind borrowing chords from the parallel minor, you will know that the Ab and Bb chords are borrowed from the key of C minor, both are common substitutions in major keys.

So if you were to take one thing from this post it should be that you should try to "hear" how the song resolves (usually it's the last chord, so try that first). I know I threw some more in-depth theory at you. Don't get bogged down in that.
Only play what you hear. If you don’t hear anything, don’t play anything.
-Chick Corea