Hi Lee,
You just need some software.....
Here is what I use:
The tape deck output feeds the input to my audio card. On the PC I use an audio editing program called Sound Forge to create an audio .wav file. Be sure and sample at 44.1KHz if you plan to later burn a CD. Using the PC software I usually fade in and out the content and maybe run some noise reduction processing depending on the quality of the recording. With the .wav file edited and safely saved I convert and create a mp3 file to download to the mp3 player.
There are lots of editing and conversion programs from free to mega bucks. The key is to find one that works the way you do and enjoy!
Robert