how do I put mp3 songs on an ipod nano?

Download iTunes. Drag the songs into iTunes. Connect the iPod.

If you want to do it this way(i do because it's easiest for me): Manage content manually on your iPhone, iPad, and iPod - Apple Support

If you want to sync all the music in your library into it: Sync your iPhone, iPad, and iPod with iTunes using USB - Apple Support <-- that one just uses an iPhone for example, it will be a bit different on iPod nano, but should still work the same way.