The key signature of a song is found at the beginning line, all the way to the left. In major keys, a piece's signature can be used to find what key it is in. This is done by:
1. Evaluating if the song is major or minor (I will only show you how to find the key in major)
2. Evaluating if the song has a sharp or flat signature.
3a. If the song is sharp, find the last accidental and rise it a half-step to find the key. A piece with accidentals F#, C#, and G# in the signature would make a song in A, because a half step above G# is A.
3b. If the song is flat, find the second to last accidental. This is the key. A song with accidentals Bb, Eb, and Ab has a key of Eb. When a song has only 1 flat accidental in the signature, the key is F.