Usually a headset+mic that has a single connector is set up with a "TRRS" (tip, ring, ring, sleeve) audio connector. Meaning, there are four metal parts on the connector. Regular stereo headphones use a "TRS" connector (tip, ring sleeve) that only has three. Same for microphones.
-----._ _ _
TRRS |_|_|_|<)
-----'
-----.___ _
TRS |___|_|<)
-----'
In order to use a TRRS connector with most USB sound cards, you will need an adapter that turns the single connector into two TRS connectors, one each for mic and audio. These are found in just about any electronics or audio/video store. Just make certain it is the right kind, as there are several different ways one can split or combine similar connectors. You want it to specifically say that it splits a combined stereo+mic into two connectors, rather than simply saying it's a Y-cable or something like that.