I had to read your post about 3 times before I figured out what I THINK you want to do - I think you're trying to take the headphone out of an older stereo, into your sound card, for the purpose of making your vinyle records into CD's -
If that's what you're trying to do (only thing that made any sense the way the post is worded) there are a couple of possible options:
The BEST way, if the turntable you're using is a SEPARATE unit, would be to go back to Radio Shack and ask for a PHONO PREAMP - RS sells one (or used to, anyway) for about $15-25, that has two RCA jacks in from a turntable, and two RCA jacks out, normally used to go into a stereo AUX input. This unit has the required RIAA equalization circuit built in, so your records don't sound like they're being played through a cheap telephone. You would also need a pair of patch cords that have RCA connectors on one end and whatever the LINE or AUX inputs to your sound card need. IF your soundcard's Line or Aux inputs are on a stereo mini jack, there are "Y" cables that have a stereo mini jack on one end and two RCA connectors on the other.
The above way will get you the best sound - however, if the turntable on your system is BUILT IN and has NO SEPARATE outputs direct from the turntable, such as RCA connectors, then you may HAVE to use the headphone output. You may be able to just turn the main volume control down enough to get the right level for your sound card (make sure you're using the AUX or LINE inputs, and NOT the MIC inputs) -
If the last way is your only choice, and you CAN'T get the levels low enough, Radio Shack also sells some little red adapters that have 30 dB pads built in. The ones I've seen are mini plugs, so you would need to find enough cables/adapters to go from whatever size stereo phone jack your headphone output is, to a pair of mono outputs ("Y" adapter), then through one of these 30 dB pads for each channel, and finally into whatever adapters you need in order to go from the mini jack on the pads into your sound card.
If you go this last route, you will probably need to raise the volume control slightly to get enough output for your soundcard's LINE or AUX inputs, but that's OK. Just make sure you DON'T try to use the MIC inputs of your soundcard EVER, because you will be going through the world's crappiest excuse for a preamp on the way to CD - The quality of signal that way could concievably be bad enough to make your vinyl sound worse on CD than it does from the turntable.
Hope I got the question right - if not, post back... Steve