A couple of things to check (I'll try to attach some screenshots, but these have been taken on a Linux box so may look a little different - options should be in the same places)
Under EDIT > PREFERENCES > AUDIO IO
Make sure you have the correct input device selected for your sound card
Under EDIT > PREFERENCES > QUALITY
Make sure the sample rate is 44100hz and at least 16bit (I would not worry about the 24 or 32 bit options if you have them, not for copying cassettes)
When you have recorded the cassette and done any editing you want use the FILE > EXPORT menu and select MP3 (or the format you want to export the file as). There is a OPTIONS button nearby. Click this and set it to 256k (or 192kps if you want a smaller file).
I personally would recommend that you have the LAME encoder library installed. To my old ears it just sounds better. I've been fortunate enough to run mp3's encoded with lame on 10K plus sound systems in clubs and venues, and they pass my taste test :-) Politics says that LAME does not come with Audacity, but there is usually a button (or help link) to the library.
If after checking this you still have 'pants' sound to your cassette rips, make sure you've not changed the mixer settings on your soundcard so you are picking up microphone or other noise for 'open' inputs.
Hope that helps you fella :-)