On a metronome you set the ‘BPM’ or ‘Beats Per Minute’. Occasionally the BPM is marked on music.
If you don’t know the BPM you can determine it using one of many apps or by using TempoTap.com. Here is what you do…
Determine the BPM of a song’s tempo.
Use the space bar to tap each beat. The BPM (beats per minute) will be automatically computed to determine the tempo of a song. The more beats you provide, the more accurate the resulting BPM will be.
There are separate directions for using touchscreen.
Here are manual instructions for determining BPM of a song via WikiHow.com.