mp3 player for 4.2.1

That's a 2nd generation iPod Touch. You can add music to it by using iTunes (the programme, not the iTunes Store). So install iTunes on your computer, add music to it (to create an iTunes Library, your Library) and then Sync the iPod with your Library.

The built-in music app on your iPod will play mp3s.

It is unlikely that you will find any apps in the store that work with that model of iPod (due to its age), but as stated, the music app that came with the iPod will play mp3s.

If you need help with iTunes. let us know.

Mp3 player on iPod can't play iTunes songs, these songs will have to go through tools like TunesKit Audio Converter to be played on your iPod.