Fraudulent purchase

has your credit card been charged? Are you sure the message is legitimate? Does it contain a link for you to sign in somewhere? It may be a scam. Check your purchases history.

Purchase History Review.  

Apple will always address you by your name or the name they have on file for you, not Dear Customer, Dear Client or by using your e-mail address.  The e-mail will be from @apple.com or @iTunes.com. E-mail addresses can be spoofed. You can go to Mail/View/Message/Show all Headers to see more. Apple emails won't have poor grammar/misspellings. Apple e-mails will never contain an attachment. Apple will never request personal information by email such as Social Security numbers, your Mother’s maiden name or full credit card numbers . 

The only exception to the above I have noticed is if you order something from the Apple Store (apple.com), your receipt will be addressed to Dear Apple Customer. That is a receipt for a purchase you initiated.

Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams.   

Identifying legitimate emails from the iTunes Store.     

Send the e-mail to Apple as an attachment to a new e-mail before deleting it. You can forward as an attachment by going to Mail/Message/Forward as attachment. Or control - click on the email and select Forward as attachment. Make sure you send it as an attachment to a new email. If you just forward it, it will probably be rejected. You won’t receive a response.

reportphishing@apple.com

I think the email is a scam. But, check your purchase history for the App Store or iTunes Store https://support.apple.com/en-us/HT204088 to make sure the charge is not from Apple. If you think you are owed a refund, see Request a refund for apps or content that you bought from Apple https://support.apple.com/en-us/HT204084.

If you decide the email is a scam,

Is it possible that you gave potentially damaging information to someone? If yes, you'll need to follow up, for example, with a credit card company, Apple, Social Security, your bank, etc.

Is it possible that you gave you Apple ID password to a scammer? If yes, then 

Change your Apple ID password https://support.apple.com/en-us/HT201355

Here is What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

To be proactive, learn about phishing, scams, and Apple

practices, etc., see:

If you see apple.com/bill, itunes.com/bill, or an unfamiliar charge on your statement https://support.apple.com/en-us/HT201382

Identify legitimate emails from the App Store or iTunes Store https://support.apple.com/en-us/HT201679

How to avoid scams when using Apple Pay to send and receive

money https://support.apple.com/en-us/HT208226

This scam related information from Apple including reporting scams to Apple see Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams https://support.apple.com/en-us/HT204759

How to identify scams related to purported apple notifications—See what Eric Root says above.

While scams and activities that compromise an account are different, learn about/Increase your security for safety. See:

If you think your Apple ID has been compromised https://support.apple.com/en-us/HT204145

Increase the Security of your Apple ID https://support.apple.com/en-us/HT201303

iCloud security overview  https://support.apple.com/en-us/HT202303

Ways to keep your information safe on Mac https://support.apple.com/en-gb/guide/mac-help/mh11402/mac

Use Two-factor authentication for Apple ID https://support.apple.com/en-us/HT204915