What to do if iCloud account is hacked

Use a strong password, one that is not a word or even based on a word. Include case changes, letters and numbers and special characters. Do not share that password with anyone, ever, for any reason. Store it in your iCloud Keychain or whatever password manager you use so you don’t forget it.

And enable 2FA on your AppleID -> Two-factor authentication for Apple ID - Apple Support

Change your iCloud (Apple ID) password.

Knowing your password is the only way anyone can get into your account.

https://support.apple.com/en-us/HT201355

Another article for you to review; it reinforces ALL of the SOLID recommendations already provided in the replies:

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