You need a line-in socket on your PC -- separate sound cards have them, but I'm not sure if on-board sound comes with one so you'll have to look around or check your handbook. Then you need a line-out on your tape player or, failing that, a headphone socket on your tape player. Connect the tape player socket to the PC line-in socket using a cable with the appropriate plugs, which you can get from most computer or hi-fi shops.
Then you'll need software to do the actual recording. I use what came with my Soundblaster sound card, but I'm sure others will recommend something if you don't have anything.