Slight delay here... quick cable-connect scenario...
On the back of the Cassette Deck, it has two outputs to Speakers, yes? One is a left, one is right/ These will either be RCA plugs (like the picture) or the little mini-jack connectors (similar to computer plug-in's). Then the other end NEEDS to be a "stereo jack". Radio Shack, Best Buy, most electronics retailers will sell something like this in the $5-10 range.
If you can't get a Stereo one, well, your resulting 'converted recordings' will be mono. Not terrible. You might even consider it "practice", too.
I can test my cabling into my computer by putting the Cassette Deck into PLAY and then I can hear it over my computer's speakers.
If I can't, then I try plugging it into a different connector plug.
If you can post details about your computer or your sound-card, maybe I can post pictures with more specific cabling options for you, but it may be later Sunday afternoon.