(You may want to read the articles about Scales, the Major Scale, or Key Signatures first.)
There are 12 different Major Scales: One with no sharps or flats, 4 with sharps, 4 with flats, and 3 with either sharps or flats depending upon enharmonic spelling.
Major Scales with no sharps or flats: C Major
Major Scales with sharps: D, E, G, A
Major Scales with flats: E-flat, F, A-flat, B-flat
Enharmonic Major Scales: C-sharp/D-flat, F-sharp/G-flat, B-natural/C-flat
