How do I finish setting up my iTunes

Log into your account in the App Store/iTunes and you should be able to change/verify it there. Or you can click the blue Edit payment methods in the article below. When signed in and the page loads, you will be on the credit card page. After updating that, click the Go Back button at the lower left and you will be on the address page. If you have a subscription, you need to add the new card, and then promote it to the primary payment method before you can remove the old card.

Apple ID payment information - change.  

Apple ID has not yet been used with the iTunes store. Please review your account information

Finish in what way? What is missing? What are you trying to accomplish?

What happens when you try? Error message(s)? More information would help.

Great