There is no such thing as an "admin user" in Firebase. You will have to create the logic for that in your app. How to do this, depends on the logic of your app (and for example the Firebase features you use).
If you're using the Firebase Database, these are good places to start:
- The Firebase Database documentation on user based security
- This gist about role based security
- Q: how do I implement role based access control in firebase
- Q: Firebase: set security rules depending on user roles
- Blog post: Implementing Roles in Firebase
- Most of the top search results here
