Build simple android app that allows two users to connect through Firebase

I'm new to Firebase and could use some help setting it up. I need an app that has 3 activities: 1) the first activity will be the "login" activity. The user will be prompted to enter a username then hit "sign in". If the username is not in use, proceed to second activity. 2) the second activity will be where players can create or join an existing room. I'd like this to work with a single join/create button if possible. Rooms will have a maximum of 2 users in it. When the user hits the create/join button the app will check to see if any room exists with only 1 person. If a room with 1 person exists, the user will be added to that room. Otherwise create a new room and add that one user. If a user if creating a room the app should show a loading screen until a second user connects to the room. 3) the third activity will show a chat box where the two users can write messages to each other. Please let me know if you have any questions or if there's anything above you don't understand. Type the word "elephant" in all uppercase in your bid so I know you've read the project details completely. Thank you!