I am a bit new to this, but this is one reason why I did not use a mac!
VST synths are great and many are free
You may need a "host" program to run the instruments
Your host can also be something like cakewalk or acid
Also, a good controller can help to input a good realistic performance
Some pro piano VSTs can be hundreds of bucks, just warning you - hope you are not on protool$!!!