Songs written in the key of C major do not require a key signature, as the key of C does not contain any sharps or flats. The following song is in the key of C major.
Here is a traditional folk melody in the key of C major. Once you have learned it, try transposing it to the keys of F and G.
Although this song uses notes found in the key of C major, it is written in the key of D minor and is based on the Dorian scale or mode, which is derived from the C major scale.)