Directly on the iPad you can only download music from iTunes - which could be your own music if you are using iTunes Match.
You can sync music to the iPad's Music via your computer's iTunes, which you couldhave bought from other sources and/or copied from your own CDs : syncing music.
If you use iTunes and have transferred/imported all your CD/tape/record collections to digital version into iTunes, you can transfer and sync all of your own digital music to your iPad.
If you do not have a computer with iTunes on it with your own music saved to it, then you have to purchase music from the iTunes Store.
If you just want to listen to music you like can go to the Apple Apps Store on your iPad and download an Internet radio app like Pandora or Slacker and pay nothing.