I don't know much theory either, but to work out the key, it's generally the note you start with and the note that sounds best to end with.

Thats the easiest method, but it isn't always the case.

For example:


Thats in the project key of A.

I hope I made sense :\
^That's not necessarily a great example, seeing as power chords are tonally ambiguous.

To find the key, you look at where the tonal center is; pay attention to the first and last chords, and see where different cadences resolve. Then, if you know your keys well, you'll be able to see what notes you're playing and figure out which key you're in. For example, if your piece contains the notes A B C D E F# G, that suggests either G major or E minor. Then, figure out where the piece wants to resolve to get the tonal center.
It depends on a lot of factors. But the decisive one is which note or chord it resolves to. Other factors: starting note/chord, ending note/chord, starting/ending Bass. Take a look at the progression. If it's a typical A minor progression like Am, Dm, G, E7 or stuff like that, then you can guess it'll be in A minor.
If you have a good sense of pitch, it's the note that sounds best to resolve or end the song on. At any point in the song.
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I use scales pretty often.
But mix it up.
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theres a sticky here for theory if you didn't know -

but basically i just try to hear where everything is resolving to. you can always hear one note that sounds like it's bass for every other note, if that makes any sense.
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