Somebody is using my account without my authorization. How do I change this

You might want to consider changing your password and making sure it is a strong one. You can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.

Link to contact Apple Support in the article.

Apple ID has been compromised

 Apple ID Password Change

Note the restrictions in the article.

Apple ID Name 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.

 Apple ID -Two-factor authentication

 Two Factor Authentification availability

 Apple ID - Using app-specific passwords

Apple ID - Two Step Verification App Specific Passwords

Apple ID - Two Step Verifications FAQ

If you refer to your Apple ID account, you could change the Password;

that should be adequate to secure your access to most things Apple.

With two-factor security, in addition, in your Apple services & devices.

• If you think your Apple ID has been compromised - Apple Support

https://support.apple.com/en-us/HT204145

How anyone can be using your account? You could put the stops to that.

• Security and your Apple ID - Apple Support

https://support.apple.com/en-us/HT201303

"Security and your Apple ID. Use a strong password for your Apple ID.

Apple policy requires you use strong passwords. Make answers to your

security questions hard to guess. Protect your account with two-factor

authentication."

• Two-factor authentication for Apple ID - Apple Support

https://support.apple.com/en-us/HT204915

• Switch from 'two-step' verification to 'two-factor' authentication - Apple Support

https://support.apple.com/en-us/HT207198

• If you forgot the answers to your Apple ID security questions - Apple Support

https://support.apple.com/en-us/HT201485

• If you forgot your Apple ID password - Apple Support

https://support.apple.com/en-us/HT201487

https://duckduckgo.com/?q=secure+apple+id&t=ffsb&ia=web

• How to secure your Apple ID | TechRadar

https://www.techradar.com/sg/how-to/how-to-secure-your-apple-id

"Your Apple ID is of vital importance. It's your link to everything Apple."

{..there is lesser 'two-step' choice that is not secure as 'two-factor'..}