We had a computer switcheroo this summer, and because my mom has an iPod Shuffle, there was some anxiety in getting the music all back. (If you don't own a Shuffle you might not know that the device is assigned to one computer at a time. Other devices are able to use up to five computers.) I imported her purchased material to the new computer (which we authorized).
If you want all of your purchases on one place, this is how. You'll need your iPod, and it will need to have all of the content you want to transfer.
1. Open iTunes. If you don't have iTunes, install it. (You'll need to visit apple.com/itunes.)
2. Make sure your computer is authorized. If you've already made a purchase from the iTunes store your computer is authorized. If you haven't click the ADVANCED tab. Click AUTHORIZE computer.
3. If your device is synced with another computer, or is an iPod Shuffle, plug it in to your computer. Click TRANSFER PURCHASES when the dialogue box pops up.
I don't sync my iPod and I manually manage my content. If you do this, plug it in to your computer. When the device appears in iTunes click FILE. Then click TRANSFER PURCHASES FROM [device].
4. Wait for the transfer to complete before ejecting your device.
iTunes How To