To connect your stereo receiver or radio to your computer sound card, you will need one of the cables shown in the photo below. The cable can be purchased at an electronics store (e.g., Radio Shack) or a merchant that supplies home entertainment products. You can purchase online the first cable here, and the second cable here. The Belkin part numbers are F8V235-12 and F8V203-12, respectively. For high end, very low noise and interference (and more expensive) cables, take a look at this cable and this one.

For a stereo receiver, you will need a 3.5 mm stereo plug to 2 RCA plugs cable shown on the left of the above photo. The RCA plugs connect to the audio output jacks of the receiver and the miniplug end connects to the Line In jack of the computer sound card, as shown in the figure below.

For an ordinary radio with a headphone jack, you will need a 3.5 mm stereo plug to 3.5 mm stereo plug shown on the right of the above photo. Place the radio at low volume and connect one end to the radio headphone jack and the other end to the Line In jack of the computer sound card, as shown in the figure below.

Connect one end of the audio cable to the input jack of the computer sound card and the other end to the stereo output jacks or radio headphone jack. If you are using a radio, place the volume control at low volume. To minimize interference noise from the computer, especially for AM stations, it is recommended to place the receiver/radio as far as possible from the computer. To accomplish this, the cable length should be a minimum of 12 feet.