How to create web application using Qt? Please advice me. Please, give a small example too.
The functionality you are asking for does not exist within Qt itself. However, there exists (at least) one third party library that allows some of the Qt code for a desktop app to be re-used to a certain extent for serving up a web app:
However, this does not magically allow you to write a QML interface with QML Widgets and have a visual interface accessible via a web browser.