The page that you posted from tells you how to create an iTunes account, or there are instructions on this page if you want to create one without giving credit card details : http://support.apple.com/kb/HT2534 . To use that new account your wife can tap on the id in Settings > iTunes & App Store on her device(s) and log out of your existing account and she can then log in with the new one.
All of the content that you and/or your wife has downloaded on your account will remain tied to your account, so only your account will be able to download updates to those apps.