How do I know which codecs I need to download?

I have a file-sharing program and until recently, everything I downloaded, I was able to watch with no problems. After a recent update to my windows media player, I can no longer watch the files I download because of the codecs. WMP says that it is aquiring codecs but none is found... I went to the codecs webpage and there are hundreds of codecs...How do I know which codecs I need to view the files I have downloaded? And where do I download them from?