You have to buy a set of PC speakers which includes a sub-woofer, they will all be inter-connected so they work together, all you have to do is plug them in to your computer's speaker-out jack(s).
I bought a 2.1 Sony PC speaker set because my sound card doesn't support any more than 2.1 (2 side speakers plus 1 centre sub-woofer) but your PC may support 5.1 or 7.1. You'll have to check the specs of your audio card.