I forgot my Apple ID on iPhone 7

You need the old ID to sign out of the phone.

Have you checked the various tabs in the Settings app to see if the ID shows there?

See if you can recover the ID/password.

Apple ID - How to find your Apple    An iCloud e-mail address is an Apple ID.

Apple ID - Find              

For the Apple ID password, enter the Apple (iCloud) ID and click Forgot Password. See if you can get an email option.

Apple ID - Manage