Play through the music first without using the metronome to familiarize yourself with the piece without worrying about time.
Start off at a moderately slow pace to start, such as at 80 beats per minute for eighth notes or 120 beats per minute for quarter notes.
Isolate portions of the music that you find more difficult to play than other areas. A metronome will help you find them and will keep you from slowing down to play the part correctly. Practice these areas until you have them down and return to playing the whole piece at the same speed. The music sheet designates the tempo in certain portions of the piece by using words like "presto" -- which means very fast -- or "adagio" -- which indicates a slower tempo.
Speed up the metronome by a few beats per minute as you get better at the song and are able to keep up with the beat.