How To Use a Metronome ???
     Most sheet music and instruction books have a "metronome marking".  This is the recommended tempo for the piece your are practicing.


All of the above examples have a tempo of 60 beats per minute. Simply set your metronome to 60 and try to play at this tempo. Note, that you can only play as fast as your hardest measure. If the hardest measure can be played at 52 beats per minute, set your metronome to 52, then gradually increase the tempo until the entire piece can be played at 60 beats per minute or more.
     There are other markings - Italian markings - these give a general range of tempo:

Largo 42 - 66 Lento 52-108 Adagio 50-76 Andante 56-88
Moderato 66-126 Vivace 80-160 Allegro 84-144 Presto 100-152

Many metronomes come with a chart of the Italian Markings above and can be set according to the music.

Types of Metronomes

Keywound metronomes:
  
These metronomes are usually sitting on top of a piano - made of wood or plastic  with a swinging pendulum let you see the motion of the beat. There are special  bell models that  will also accent the first beat with a bell sound. The keywound models give the most natural sound of all the metronomes and are very pleasing to the ear. Winding most  keywound model 8 to 10 turns, will run most metronomes for at least 30-40 minutes. The tempo is changed by sliding a weight up and down the pendulum.
  Advantages: Great visuals as the pendulum can be seen swinging with the beat. Simple operation, easy to use. Great acoustic sound and classic look.
  Disadvantages: Most models are not portable. Will not stand much abuse (dropping or bending of parts).

Quartz Dial metronomes:
  
These models have a dial on the front of the metronome that will quickly let you change the tempo by simply turning the dial. There are many features available. Volume Control - many times you do not need full volume and would like to turn the volume to very low. Flashing lights - all have a single light that flashes with every beat, some have many lights that swing as a pendulum type metronome above. Sound - most have a wood block sound. Accenting - will have a different sound on the first beat of each measure, such as a bell.
  Advantages: Size - many are small enough to fit in a pocket. Volume - many models give a very loud click or very soft (using volume control). Accuracy is very high, all the time.
  Disadvantages: Most models use a 9 volt battery that will eventually need replacement ( most last at least 100 hours of use or more).

Why Use a Metronome ???

Metronomes:
Wittner
Seiko
Matrix
Korg
Dr. Beat
Cherub
Feel the Beat
Intelli

Further Links:
Techniques
History