The melodica is a versatile and highly portable miniature hand-held piano. It is played by simultaneously pressing down keys and blowing air into the mouthpiece. Playing the melodica is very similar to playing a standard piano, and if you can play the piano you can almost certainly play a melodica. For those who don't have a piano background, however, learning a few simple techniques can produce some wonderful music.
Consult a chart of the notes on a standard piano. The keyboard of a melodica is a shortened version of the full keyboard. Identify particular keys of the melodica that correspond to particular musical notes.
Blow into the mouthpiece while pressing down on different keys. Play around with the instrument and get comfortable holding it. The right hand should be closest to your mouth, with your left hand near the end. Both thumbs should wrap around the bottom.
Play a C major scale. The C major scale is played solely on the white keys, and consists of the notes C, D, E, F, G, A, B, C. Try to compose a little melody using those notes, beginning and ending the tune on a C.
Try a C major chord with the fingers of the right hand. The notes of a C major chord are C, E, and G. Hold all three of those notes down while blowing air into the mouthpiece.
Play a melody using the notes of the C major scale with the left hand while playing a C major chord with the right hand.
Move the fingers of the right hand up one white key each from the C major chord to play different chords. This can produce chords all the way up the melodica keyboard, namely the C major, D minor, E minor, F major, G major, A minor and B diminished chords. Continue to play individual melodic notes with the left hand while playing different chords with your right. Practice until you are composing your own songs with the melodica.
Consult archives of piano theory for additional information. The melodica is typically two octaves of a standard piano, and so obeys the same musical principles.