ok. where is the deauthorization option in the iTunes Store menu? answers would be great! :)

okay, so Im trying to burn a disc with a specific song from iTunes, which Ive already purchased. however, when I went to download the song, it says my computer was not authorized. Am I making this harder this harder than it actually is?? another question, how do you authorize a computer to where I can burn this disc?!

thank you so much xx

k.hullander0624