Download an audio recording program such as Audacity. Windows comes with a free program called Sound Recorder, but it is not able to handle sound files longer than 60 seconds. Audacity is a free sound recording program which can handle longer files.
Look at the output on your cassette player. If you are using an old boombox, you probably have a 1/8-inch mini plug output like you would connect a pair of headphones to. If you are using a stereo, you probably have a pair of RCA outputs colored red and white. Now look at the back of your computer. There should be a mini plug with a little picture of a microphone on it. That is your input.
Get an adapter. You will almost certainly need one with a mini plug on one side and either a mini plug or two RCA plugs on the other side depending on whether you are using a boombox or a stereo. See the links below for both adapters.
Plug in the adapter, open the sound program and turn on your tape player. Select "New File" from the file menu.
Press the play button on your player and the record button on your sound program. Record about a minute of the tape.
Play back the recording by pressing the play button. If it sounds very fuzzy and distorted, the volume on the tape player is probably too high. Turn it down, delete the sound you just created and try again.
Once you get your volume right, record one side of the tape. Save the file, flip the tape and record the other side as a second file.