If your cassette deck has phono outputs then use those instead if not then the headphone socket will do. If you have a suitable lead conncet this between the cassette and your pc sound card. Most sound cards have 3.5mm stereo jacks but some higher spec ones have phono connectors, you will need to look at the back of your pc to check this.
Now open Audacity and select edit then preferences, this is where you can check the editor will record and play from the correct sources and if its stereo or mono. Most of the other prefs defaults will be fine for recording from cassettes click ok.
Back to the main window and select line in from the drop down box next to the rec level slider. Now do a test recording, play a tape and press the record button in Audacity. You should set the rec level so that the loudest sound is no more than -8dB on the vu meter. Record about a minute and then stop. You should have the audio sample displayed on screen and be able to listen back to it, if it sounds ok then save it to disc.
You can choose to save as wav mp3 ogg, if your intention is to generate audio CD's then save it as a wav file. Close Audacity then open it and make sure you can open and listen to the file you've just made. If that all goes well then you're ready to go, there is a help file in Audacity which will explain most of the things I've mentioned in detail for you and also a website with further help and tutorials here. Good luck.
http://audacity.sourceforge.net/help/documenta tion