How do I recover forgotten iPhone passcode and Apple Account credentials?

Your User Name is your email address unless you have changed things from your original set up.

If you forgot your Apple Account password - Apple Support

No one here can help. We don't know what your iPhone username or password is, either, and we have even fewer ways of checking than you do.

Fortunately Apple have a help page to guide users through this kind of problem. It's where I'd start:

If you forgot your iPhone passcode or your iPhone is disabled - Apple Support