First thing, I think one of the issues the Yamaha YDP-181 has no USB to host.
You can either buy a Midi to USB adapter and connect your piano to a USB on the PC, a quick check would seem to suggest this one for the yamaha, http://www.amazon.co.uk/Yamaha-SUX1...8-1&keywords=ux16+usb-midi+interface
or perhaps any one of the cheaper ones would do, check the manual for your piano. Others will be able to comment on a decent cheaper adapter if one exists, I have no experience using such adapters.
The adapter approach is a good option if you feel your current onboard soundcard is good enough and may be your cheapest route since there is a good chance you will find that it is good enough if your PC is you fairly typical i3 - i7 machine these days.
Alternatively, you could buy an external USB audio interface with MIDI inputs, a fairly decent cheap option that is well regarded would be the focusrite 2i2 which has MIDI http://www.amazon.co.uk/Focusrite-S...8947&sr=8-1&keywords=scarlet+2i2
another route: Internal PCI soundcard which has MIDI input, but those can get really costly.
With both you'd need a midi cable to connect the piano to the MIDI inputs of the interface/soundcard, but that type of cable should cost hardly anything.
page 52 has the relevant info.