hello guy

right now i try to develop a very simple app that can transfer message with other device.
this app using same network to allow them to sent and receive message whatever they recognize them.
this app still can transfer message using wifi as access point without internet connection. the app should
be like this https://www.asus.com/zentalk/thread-25939-1-1.html
the problem is i do not know how to start it. if there is any good tutorials or website, feel free to share with

thank you.