- PC or Mac. I use a PC laptop. It's old and relatively slow, but still works quite well. Any newer unit will serve even better, so no worries there.
- Sound system. You could plug headphones into the PC, or you can feed the PC sound to an amp and speakers.
- Piano library software for the PC.
- Upgrade sound card for the PC (maybe).
The basic setup looks like this: Piano Wiring 1
This uses a USB connection between the piano and the PC. This is possible for newer pianos that have a USB connection.
This diagram shows additional ways to hook things up: Piano Wiring 2
This shows three possible ways to connect the piano to a PC: USB, MIDI-to-USB, and MIDI. (The latter requires a PC sound card with a MIDI interface.)
This diagram adds a mixer to allow you to listen to either the PC piano sounds or the piano's native sounds, or both: Piano Wiring 3
And this one adds equalizers to tweak the sound, and a DVD player to allow play-along with an audio CD: Piano Wiring 4
This is what I use. I already had the piano, PC laptop, and headphones. I bought the rest of the equipment used on Ebay and Craig's List for around $375.
For piano software, there are many choices. Do an online search for "piano libraries" or "VST pianos" or something similar and you'll find many possibilities.
Also use this forum's search box to find threads in which this topic is discussed.