Closing Apple and iCloud accounts

If you mean you want to delete your Apple Account, you will need to follow the steps here and read it carefully. Make sure you are not currently signed into that Apple Account on a device, otherwise you will need to use the Activation Lock removal request since the Password to remove the account from the device will no longer work.

How to delete your Apple Account - Apple Support

Just so it is clear, an Apple Account and an iCloud account are the same thing. iCloud is just part of an Apple Account.

You may also wish to read this article for deactivating account which is more reversible should you later decide that you may wish to use it again. How to deactivate your Apple Account - Apple Support