Knowing how to read the key signature to determine what key to play is critical to reading music, but it isn’t difficult. Count the number of sharps or flats in the key signature, and then you can use the circle of fifths (or the following table) to determine which major key to play in.

If the Key Signature Has The Music Is in the Key of
1 sharp G
1 flat F
2 sharps D
2 flats B flat
3 sharps A
3 flats E flat
4 sharps E
4 flats A flat
5 flats, 7 sharps C sharp/D flat
6 flats, 6 sharps F sharp/G flat
7 flats, 5 sharps B/C flat