You'll need an optical cable to connect your computer to the receiver. Only use HDMI if you want video to go to the receiver as well. Pick the optical cable with the right connectors on this page:
Audio Cables - Optical Toslink
Chances are both ends are regular Toslink and not miniplug. Some sound cards have a miniplug end. The receiver end is regular Toslink.
You'll also need some speaker wire. Any sort of 16-18g copper wire is going to be fine. I use the 16 gauge in-wall stuff here:
Speaker Wires
I use the in-wall because it has a white jacket with red and white inner cables. It looks much better than the clear-jacket stuff.