How do I deactivate my old computer so I can use this one
If you are asking about authorizing your computer to use your iTunes Store account, you can have up to FIVE computers authorized to use one iTunes Store account.
To deauthorize a computer, run iTunes on that computer and log in to your iTunes Store account. From the menu bar, select Store -> Deauthorize This Computer. The command to Authorize This Computer is at the same place; do this from your new computer.
NOTE: You can have up to five computer authorized per iTunes Store account, so you can have both computers authorized.
If you no longer have access to that old computer, please post back.