For heavier passages use the palate attack, using air as in whistling either more or less, as required. In this way suitable articulation for all kinds of playing will develop. Think of it as blowing into a balloon. It's an instrument not a birthday cake. The more notes you play at the same time the softer the instrument may sound as you are displacing air thru more reeds.
Ethnic music works well.
The strong melodies fit well in a 3 octave range. To start think of simple melodies. Be aware of the range of your melodica and be prepared to transpose or change octaves to accommodate your song. My suggestion is to start with scales and play against a metronome . It will seem different at first.