You have to code your own app Elham. Here is an example of a chat app you can modify to your requirements
How to: A Family Chat app built with the MIT CloudDB component
Download the aia and try the basic app or download the simplified aia. The example shows what is possible using App Inventor and a CloudDB or a FirebaseDB. How is the app you want to build different from the examples?
To test the app, you have to compile the app and load the apk on to two phones to ensure they are communicating.
You did not answer the question above. No one can provide meaningful advice unless you communicate. No one will build your app for you. Learn to code. Try some things. When you have a specific problem, post your code Blocks here and someone might be able to provide specific advice.