Someone is using my Apple ID

See these security-related documents, especially the first two.

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

Security and 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

Using app-specific passwords https://support.apple.com/en-us/HT201355

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

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Also assuming you already are using two-factor authentication, you can Check your Apple ID device list to see where you're signed in  (https://support.apple.com/en-us/HT205064) and remove any device that shouldn't be signed in using the steps in the link.

If there are none that shouldn’t be signed in, then you should not have a problem at this point. But, if an unauthorized device signed in, 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

Hello Jenniferbragg19,

Go to https://appleid.apple.com/account/manage.

Click "Change Password" in the Security Section.

Change Your Password to a password that you have never used and do not use.

Follow the instructions on https://support.apple.com/en-us/HT204071 to sign in to all of your apple devices and/or services.

Hope this helps!