How do I set up a new email account
How Do I set up a new email account with Mail besides the one I already have
See -> How to create a new Apple ID
How to create a new Apple ID – Apple Support (UK)
See this Apple article: Sign in with your Apple ID
Sign in with your Apple ID - Apple Support
👉 Use one Apple ID to access all Apple services and make all of your devices work together seamlessly.
You can't merge multiple Apple ID.
See also this Apple article: Change your Apple ID
Change your Apple ID - Apple Support (SG)