How do I add music to my ipod on my computer without deleting old music on my ipod?

How do I download music from my computer to my ipod touch without deleting old music on my ipod?

Details: I've tried downloading songs to my ipod that i got from the Itunes store but then it say's it will delete all the other music and only leave the one's that im downloading. (p.s. On my computer I don't have the same music as on my ipod.)