My Apple ID is being used by someone else

"If you forgot the answers to your Apple ID security questions" click here --> https://support.apple.com/HT201485

If you see the message "not sufficient information" see if at this stage you can still sign up for Two Factor Authentication:

Two-factor authentication for Apple ID --> https://support.apple.com/HT204915

If you cannot resolve the issue by signing up for Two Factor Authentication, "Contact Apple for help with Apple ID account security"; click here --> https://support.apple.com/HT204169

Check your device list to see if there is a device signed in that you don't recognize. You can remove the device using the steps in the article below. As a precaution, change your password.

Apple ID device list -Check to see where you‘re signed in.   * 

 Apple ID Password Change.   -   

 What to do after you change your Apple ID or password - Apple Support  -   I suggest doing this before you change your Apple ID or password.