Input selection

Check under Audacity's software preferences to make sure the recording input is set to where the tape deck is connected (mic input, line level input, USB audio adapter input, etc.). If you're using the mic input on your computer, turn the Mic gain here all the way down to line level.

It's better to have a slightly quiet recording that you can boost later than having something recorded too loudly. Distortion caused by overdriven volume can't be undone.