Decide What Suits You Best
Step1
Start with two premises: first, a synthesizer only creates electric signals, which you'll generate by pressing down any key on the piano-style keyboard. Second, if you're only a beginner, you may want to start with an analog synthesizer, which allows far greater customization of sounds than its digital counterpart.
Step2
Once you've formed chords with your right hand, spend some time developing rhythmic bass lines with your left hand.
Don't worry if your keyboard technique is basic to nonexistent. Try humming your melody line--whether it's someone else's song, or your own--and match that tone on the keyboard. Hit one note at a time, if necessary, until you've worked out the whole line.
Step3
Now review the different tone controls you'll use to manipulate your sound. In general, the most important are the attack, release and resonance controls, which determine how long a particular keyboard tone will sound. Depending on your preferences, abrupt cutoffs or lengthier tonal clusters can dramatically change a song's mood.
Step4
Introduce some greater variety to the proceedings by adding a sustain pedal to your sonic arsenal; why should guitarists have all the fun in this department?
Experiment--Then Make Some Decisions
Step1
Tweak preprogrammed sounds--of which you'll have anywhere from 50, to 60, 100 or more--by seeing how quickly you can toggle between them. This approach will help you build speed and dexterity, particularly for heavily syncopated or fast triplet rhythms (for further reference, see Dave Greenfield's approach on the Stranglers' "Tank").
Step2
Don't settle for the same tones on every song; look for different shades of expression in the same groups of settings, such as on orchestrations. Remember, too, that an analog synthesizer is monophonic, meaning that it only plays one sound at a time. To get the opposite effect, you'll need a polyphonic approach, which requires a digital synthesizer.
Step3
Consider whether you want to use the synthesizer as a non-melodic instrument--as the Band's Garth Hudson did in his brain-rattling concert workout, "Chest Fever"--or a melodic punctuation device of the kind featured extensively on the Who's epic double album, "Quadrophenia" (1973). Only your imagination can limit the possibilities--as horror director John Carpenter so memorably demonstrated with the cold, metallic textures of his scores for "Halloween" (1978), and "The Fog" (1980).