If you restore your iPhone from a backup, your iTunes account log in will be restored along with your other data (though you may be asked to enter it). Use the existing iTunes account/Apple ID that you've been using with your iOS devices.
If you choose to set up your iPhone as new, though, you'll be asked to either log in to, or create, an iTunes account/Apple ID. Either log in, or follow on the onscreen instructions to create a new account, and proceed.
