Sorry your question doesn't make sense. What do you want to do? Buy songs and make sure they are in MP3 format? or rip songs off your CDs into MP3 format? Please elaborate.
Amazon Music has free songs you can download using their downloader which will convert them to song.mp3 for you. I would recommend you doing that to see how easy that works. Than attach your MP3 player remember to make sure that USB mode for HDD is active otherwise it won't work right.