The key signature at the start of a line of printed music announces both what sharps or flats are being used in the song, as well as what key the song is in. If there is just one flat in the key signature, the flat is always Bb. Two flats in the key signature are always Bb and Eb. 3 flats in the key signature would be Bb, Eb, and Ab. 4 flats would be Bb, Eb, Ab, and Db. Then comes Gb, Cb, and Fb. You can easily remember the order of the flats by spelling the word “BEAD” and then recall the phrase “Go Catch Fish.”
The sharps in a key signature are just reverse from the flats; instead of BEADGCF, the order of the sharps is FCGDAEB. So one sharp in the key signature is F#, 2 sharps in the key signature would be F# and C#, and so on.