Select the whole song by pressing Ctrl + a. Open your audio file (by choosing File > Open). The key signature is placed immediately before the time signature. Just like a written signature tells you a person's name, a key signature tells you the key to making music. To find out, look at the beginning of the music, on the staff, just after the key, you'll see a series of flats or sharps.
To accurately calculate the BPM of a song, it is important to know the length of the measure (measure).
