circle of fifths

Your question's a bit confusing...

If there's only one sharp in the key signature of music, it's the key of G (the F notes are sharp in the key of G). If there's only one flat, it's in the key of F (the B notes are all flat in F).

If you're looking at sheet music, and there's a flat note next to a C, that's an accidental - C is only flatted in the key signature in the key of Cb (which has 7 flats). Accidentals are momentary departures from the key, and they last only until the next bar line.

If you're playing a series of notes, and trying to figure out the key from them, you need to make sure you have enough information. If the only note you're playing with a sharp is C, you probably have no F notes in the melody - if you did, they'd be sharped as well.

Key signatures go like this:

No accidentals = C major/A minor
1 sharp = G major/E minor
2 sharps = D major/B minor
3 sharps = A major/F# minor
4 sharps = E major/C# minor
5 sharps = B major/G# minor
6 sharps = F# major/D# minor
7 sharps = C# major/A# minor
1 flat = F major/D minor
2 flats = Bb major/G minor
3 flats = Eb major/C minor
4 flats = Ab major/F minor
5 flats = Db major/Bb minor
6 flats = Gb major/Eb minor
7 flats = Cb major/Ab minor

Greybeards got it! Basically you need to know if it is an accidental or an acual key. The beginning of the song should at least tell you, if not every staff. Just look all the way to the left after the clef that will tell you. once you know that then your in bussiness.