Can you remember the phrase 'BEA do good cheap flights'?
That's the order of flats for your key signature's BEADGCF. To find out what key you are in, cover the first flat, and the next is the key. Just remember that one flat ( b ) is the key of F.
If you can remember it backwards FCGDAEB, that's the order of sharps for your key signatures. To find out the key look at the last sharp, this is the leading note, or one fret below the key note (or tonic). So for eg. One sharp F# is one fret below G. So its G Major.
Can you remember Major = - 3; minor = + 3? Then you can find the relative minor key. Each Major key has a minor key which shares the same key signature.
So for C [size=4]Major (easy because it has no sharps or flats) count back 3 - CBA - and the minor key = A minor. [/size]
[size=4]To[/size][size=4] find the relative Major, count forwards for 3, so from A minor - ABC - and its C Major. [/size]
[size=4]There's a bit more to it, but you're almost halfway there. Unless you want to get serous with theory qualifications, in which case you need to know things such as when Beethoven last washed his socks. [/size]