To play a piece of music in the exact tempo that it was written, there are a couple of things you need to look for at the top of the staff. For instance in a piece of music "I Saw Three Ships / Away In a Manger / Sussex Carol by ENGLISH CAROL, JAMES R. MURRAY and TRADITIONAL, the tempo is marked a dotted quarter note is equal to 84. If you have a metronome then slide the weight down the metronome stick until it is in line with 84. The metronome will rock back and forth giving you the perfect rhythm to count out the time to this piece.
If you don't have a traditional metronome you can go online to METRONOME ONLINE and set it!