Go to a shop and buy one. That's the easiest and cheapest way.

If you insist on developing one yourself: look for an 8051 derivative with built-in IDE interface and MP3 decoder, they exist. There is no way you can do this with the 8051 CPU alone.

If this is a pet project, I think you are about to bite off more than anyone can chew (is that the right expression?) There have been postings about this before, no-one ever reported back with a working MP3 player... (bwahahahaha!).