Q: I made a mistake deativating my new computer from itunes and now i cant reactivate

Hello there, mslynn45.

It sounds like you are just wanting to know how to Authorize your computer with iTunes. The following Knowledge Base article offers up the steps on how to do just that:

Authorize your computer in iTunes

How do I authorize a computer?

1. If you don't have iTunes on the computer you want to authorize, download and install the latest version.
2. Open iTunes.
3. Mac: From the menu bar at the top of your computer screen, choose Store > Authorize This Computer.
   Windows: Hold down the Control and B keys on your keyboard to turn on the iTunes menu bar. Then choose Store > Authorize This Computer. Learn more about menus in iTunes for Windows. In earlier versions of iTunes, you can access this option from the Advanced menu.
   
   If you're using iTunes in full-screen mode, hover your mouse at the top of your computer screen. The menu bar appears. Exit full-screen mode by pressing the Control, Command, and F keys on your keyboard.
4. Enter your Apple ID and click Authorize.

If iTunes asks you to authorize your computer again, it won't use a new authorization. iTunes uses the same authorization for the same computer.

Thanks for being a part of Apple Support Communities.

Cheers.