I agree with EC. Just connect your tape deck directly to your sound card's RCA inputs, assuming it has them. If not, you may need an RCA to mini plug adapter or cable, both of which should be easy to find.
You didn't specify whether you are using a PC or a Mac, but assuming it's a PC running Windows, you'll need to go into your settings for "Sound and audio devices" using the Control Panel, and from there select the "Volume" tab. Under "Device volume" click the "Advanced" button. It should pull up a box labeled "Volume Control." In that box, in the upper left corner choose "Options," then "Properties." A new box for "Properties" should appear. In that box, under the heading "Adjust volume for," select "Recording" and hit the "OK" button at the bottom. A new box called "Recording Control" should appear. From there, simply check the box labeled "Select" beneath the "Line In" column and close the box. If you do not see an option for "Line In," then you may have to go back to "Options" in the "Recording Control" box and select "Properties," then "Show the following volume controls" and check the box to show the "Line In" control. You then have to go back and ensure that of the recording volume controls shown, only "Line In" is selected.
The "Line In" setting on the Recording Control should be the red and white RCA inputs on your sound card, or if you don't have RCA inputs, a mini plug input (for which you will need an RCA to mini plug adapter or cable). Once you have your tape deck outputs connected to the Line In, the sound recorder in your computer should record the tape deck's output. You'll need to start up whatever software you have for recording, even if it is as simple as Windows' own rudimentary recorder, and control the computer's recorder from there.
Good luck.