Forgot password
Why does my Apple ID login with my fingerprint online but not in the App Store
To your topic question:
Typically an Apple ID is an email address you have used.
Click on this link to read what to do "If you forgot your Apple ID" --> https://support.apple.com/HT201354
or go directly to this link --> http://iforgot.apple.com/
Also read about "What to do after [or before] you change your Apple ID email address or password" - https://support.apple.com/HT204071
If you still need help:
Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."
https://support.apple.com/HT204169
To your other question:
Use Touch ID on iPhone and iPad - Apple Support
If Touch ID isn't working on your iPhone or iPad - Apple Support