To start off -
Know your Apple ID and password, the One account for everything Apple. Keep this information safe and do not share with others. Do not give this information away to anyone on the phone. Do you need help with your Apple ID, read If you forgot your Apple ID password where you can learn how to to use a family member's device to update your Apple ID password.
If you do not have Two-Factor Authentication, 2FA, turned on - DO IT NOW! Read Two-factor authentication for Apple ID you will also want to use a PASSCODE to unlock all your Apple devices.
Other articles to read are Security and your Apple ID and Recognize and avoid phishing messages, phony support calls, and other scams.
Review your Apple Trusted devices on the Apple ID Support site. Remove devices if you trade-in, sell or give away.
... there will be a test after reading this material. No, just kidding. But read now and come back and read these articles again. When you need help from Apple Support you will have some background for the questions they ask and the answers they provide.
It is helpful to have more than one email account. You will have a primary email that is used for connecting with friends and family and many others. The fall-back email is one you do not share widely - and it will be used for your Apple ID Notifications or as an alternative way to contact you. The fall-back email can be used for other online accounts as well. You need to know the strong password given to this account.
Make some time to record how your credit cards are used. Keep track of the cards, issuing bank and the customer service phone number. What reoccurring payments are made and the annual or monthly schedule for payments. Print and file recent account statements. Review for unusual charges.
An Apple Credit Card from Goldman Sachs does not have an embossed number. There is a chip and behind the scenes there is a credit card number. Look in Apple Wallet on your iPhone to see it. Make a screen snapshot of this information and file it in a secure location. If you experience a security breach and this card is involved, you can request a new credit card number from GS. On an iPhone or Apple Watch, credit cards registered with Apple Pay / Apple Wallet send tokens from each point-of-sale, not your credit card number.