The Flat or Sharp symbols (accidentals) at the beginning of a piece signify what key the musical piece is in. For example, one Sharp would signify that musical piece is written in the key of G Natural because G Natural has one Sharp in it (F#). However as you read the music you also have to be careful that the song is not in the relative minor key, for example : The relative minor of G Natural is E minor. However, that's where you read the piece looking for accidentals and so forth. Moreover, if you know your scales and the Sharps and Flats in each you should have no problem.