You can do this in 3 steps. You might need to experiment to get the audio gain (loudness) right.
1. Connect your cassette recorder's output to mic input of your PC or macintosh. You might have to buy an adapter that adjusts the "audio gain" to the right level - so signal does not come in too soft or too loud. Since you are sending a slightly amplified signal into that mic jack and mics usually do not put out a lot of power, you don't want to overpower the mic input of your computer.
2. Run a program to capture the audio. Sound Recorder is the name of the one that comes with MS-Windows. If you have a different computer, check your owner's manual.
3. Play the cassette.
4. Save the audio into a file. If you can save it to MP3 directly, do that. If you can't, then try saving it to something like WAV or AIFF.
5. If the file is not already in MP3 format now, try getting your MP3 player to load it and try to get it to convert it to MP3. If you can't get it to do that directly, I can tell you a trick. Burn it to a blank CD-R, then "rip" (input/encode) it to MP3 audio format. Yes, it wastes a 50 cent CD-R but it should work and is pretty simple to do. If you do not have a CD burner in your computer, that is a CD drive that reads and writes, then you will have to hunt around the web for a WAV-to-MP3 conversion utility.
6. Now you can just reference the MP3 file from your web page. An old, MS-IE only way to do that was with the non-standard BGSOUND tag in HTML, or a Netscape-only EMBED tag. Now it looks like the in way to do it is with the OBJECT tag.
<object data="http://www.YOURSITE.com/YOURFILE.mp3" type="audio/mpeg">
You do not have a player for MP3 files defined in your web browser.<br/>
Just download this file and then open it:
<a href="http://www.YOURSITE.com/YOURFILE.mp3">YOURFILE.mp3</a>
</object>
You are going to have to put the correct host name and filename in - YOURSITE and YOURFILE are just placeholders I put in to show you where to do that.
I included some URLs for pages for you to refer to in case I overlooked something.