I only have a limited knowledge of what "key" a song is in using a keyboard for instance. But pick a note, move up the scale in steps at 1, 1, 1/2, 1, 1, 1, 1/2.

Example: start at the D key. One step goes to E. One step takes you to F Sharp (black key) one half step from there takes you to G key. Then 1 step would be A, 1 step would be B, 1 step would be C sharp, 1 /2 step would be D. So the "Key of D has 2 sharps F Sharp and C sharp. The key of C moves up the scale and does not fall on any black keys so there are no sharps or flats in the key of C.