Changing the value in the Tempo field speeds up or slows down the current song. This value gets saved and re-loaded with the song file.
You can also change the tempo by pressing your keyboard's space bar 4 times at the desired tempo.
Check the Metronome turned on box if you want the metronome on all the time.
Check the Silent when not playing or recording box if you only want it on while you are playing or recording a song.
Press the Advanced Settings tab to see more fields.
Change the values of the Synthesizer and Channel values to control which synthesizer(s) are to play metronome sounds.
Try different values in the MIDI Note field until the metronome sounds ok.