How do I authorize my computer to download previous purchases?

I have downloaded a few songs on another computer that is not my own. Now these songs appear on the iTunes music list on my own computer and I ould like to listen to them, but in order to do that I must download them again. However, when I try to download them, iTunes does not allow the process to begin. A window saying "Your computer is not authorized. You must authorize this computer before you can download previous purchases." So my question as stated above it how do I authorize my computer to do that??!