Notice that there is one sharp. That sharp symbol above to the left is the signature for key of G major. The key of G major is the only major key with 1 sharp. So if you write a song in G major, the sharp symbol is placed on the F line above to the left. If you found a sheet of music with no symbol and the beginning, it would be in the key of C major.
C major has no sharps or flats.
It is the only major key with no accidentals. Accidentals are sharps and flats and other things we will be covering soon. Use this information to construct all of the major keys. Notice the sharps and flats of each key you construct. Although G major is the only Major key with this signature, there is one minor that shares the same signature.