If you want her to have a new iTunes account then there are instructions on this page for how to create one : http://support.apple.com/kb/HT2731
Or if you don't want to give credit card details then follow the instructions on this page when creating the account : http://support.apple.com/kb/HT2534
You can then use that account on the phone (Settings > iTunes & App Store for the stores, Settings > iCloud etc)
All of your downloads from your existing account will remain tied to that account. If you want to use the same computer for syncing then you can do do, you can have more than one account authorised on a computer's iTunes (Store > Authorise This Computer menu option), but only one account can be logged in for purchasing/downloading. But if you use iTunes Match, automatic downloads or redownload an account's past purchases then you may tie that computer to that account for 90 days : http://support.apple.com/kb/HT4627 - so you won't be able to do any of those three things on a different account until the 90 days have completed (it shouldn't affect being able to make new purchases)