If you already have a cassette recorder that will plug into a computer, then all you need is software to do the conversion for you. Audacity is a program you can download for free from the internet. Once downloaded and installed on your computer, you have to configure it to work with your tape recorder.
Another option is to buy a special converter box with a cassette tape and CD deck built in. These boxes do it all for you. Just put the tape in and a blank CD and push the Play buttons and wait. No software is needed and the box doesn't have to be connected to your computer. However, the downside is that these boxes can be expensive.
There is another option, which is much cheaper. Four Corners Direct sells a "Tape to MP3" converter
for about $30. This converter plugs into your computer via a USB cable and comes with its own software (Cassette Mate). It's simple to use and works. However, there's a small catch. This device is converting your cassette tapes to MP3 format. This means that you'll have you'll be somewhat limited in listening to the contents of that tape.
You can listen to MP3 files on your computer without any problems. You can also listen to them on your MP3 players. And some CD-players (including those in cars) have the ability to play MP3 files.
So the good news is that you CAN convert your old cassette tapes to CD. It's juts a matter of finding the right solution for you.