Since the Keystation is a MIDI controller, you need some sort of sound generation program to control (sorry if this is obvious…not sure what level to start with).

I’m a professional keyboard player, and have all sorts of hardware and software synths I’ve accumulated over the years, from an old Fender Rhodes to a Kurzweil K2600xs (my main axe).

So it really depends what kind of music you want to make. Dance music? Get a module that specializes in loop stuff. Basic all-around stuff? Get a cheap standard polysynth.

But if you want to record multitrack, you’re probably talking the light versions of Cubase or Cakewalk for starters. They’ll typically have built-in synth plug ins that will work well, and you can find the entry level versions in most office superstores like Staples, Office Max and the like.

Hope this helps.