How do I go about paying my outstanding bill?

Bill for what? Read the links below about various ways to pay if these are items in the iTunes and App Stores.

- Add a payment method such as a credit or debit card to your account. See this link about 'Payment methods that you can use with your Apple ID' --> https://support.apple.com/HT202631 then see this link about how to 'Change, add, or remove Apple ID payment methods' --> https://support.apple.com/HT201266

- Add funds to your Apple ID balance - "Use a payment method, such as a credit or debit card, to add funds to your Apple ID balance." This takes the previous payment method one step farther by optionally using it to irreversibly add balance to an Apple ID instead of having the card charged each time.

Click here --> https://support.apple.com/HT208687

- Create and add money to an iTunes Pass - "At Apple Stores and select retailers, you can add store credit directly to the Apple ID that you use for App Store, iTunes Store, and iBooks Store purchases." - https://support.apple.com/HT203021

- Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - https://support.apple.com/HT201209