Whether you're in a band or just want to fool around with sounds, there's almost no instrument that allows you to sound good as quickly as a synthesizer.
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Purchase a chord dictionary to get a basic knowledge of scales and chords, if you don't have that knowledge already.
Choose your instrument. Not all keyboards are synthesizers--a true synthesizer can produce a number of sounds by generating signals of various frequencies. Playing one requires either headphones or an amp (or a loudspeaker, or P.A. system). Start by fooling around with the different settings to learn what sounds your instrument is capable of. There are an unlimited number of models available, but some features remain the same no matter what synthesizer you play.
Explore the different ways to play, depending on what type of music you prefer. Many popular songs of the 1980s typically contained an easily identifiable synthesizer hook ("Der Kommissar," "The Stroke," "Shake It Up," "Subdivisions," "Jump"). Many dance tracks used a synthesizer bass. Other occasions have called for sparkly solos. Remember, if you're playing with others, you're only one part of the puzzle.
Once you become proficient, you can start to explore MIDI (Musical Instrument Digital Interface) options, which will allow you to use your computer to have your synthesizer play multiple parts simultaneously.