Virtual instruments are great, especially for playing with in your home (or studio). Playing live on stage with them requires that you know a thing or music hardware and your computers software. It's definetly possible though.
This is the basic Idea:
You have you computer with some "viritual instrument" standalone programs or you can use them as "plugins" in another type of music software called a sequencer (a sort of host fort you viritual instruments".
Apart from having a fast processor and decent amount of ram-memory (in other words, your computer must be decent to get decent results so that the sound won't stutter ) you also need a decent soundcard.
It should support ASIO- sounddrivers (to reduce the risk of stuttering - latency) and of course a midi-in input to plug in your keyboard.
Once you plugged in your keyboard to your computer with a midi-cable you need to hear what your playing and since the sound is generated in your computer you need to connect your soundcard's audio-output to a pair of speakers.
I guess that's the basic to playing viritual instrumtens on your computer using your yamaha/korg/casio/whatever keyboard.
hope i helped some
