If you want to record from another source, like a cassette deck or a vinyl LP, then you need to do a direct connection using an audio cable. What you’ll be doing is using this cable to connect to the output on the audio source and then plugging the other end of the cable to the input on your computer. If you are on a desktop PC, then you will most likely have a ‘Line In’ jack on the back of the PC. This functions sort of like a microphone jack, but doesn’t have the amplification options. If you are using a laptop, then you’ll most likely just have a microphone jack. Either one is fine, but the Line In jack is preferred.
On the audio source, you may have a headphone jack, or you may only have RCA cables. Either way, you can go to some place like Radio Shack and get the cables and any necessary adapters. Make sure you get the right sizes, as the computer will use 1/8” plugs, but some stereo systems uses ¼” plugs. You can also get an adapter that will take two RCA jacks and adapt the signal to fit a 1/8” plug.