Arif,
Glad to hear about your interest in App Development.
If you are interested to learn about cross-platform app development using AppStudio for ArcGIS, the best place to start is the AppStudio resources page, AppStudio for ArcGIS Resources | Downloads, Documentation, Templates & Videos
It would also be very useful for you to start learning QML, which the programming language that is used in AppStudio to construct the User Interface. In AppStudio, we use QML (user interface) and Javascript (application logic), similar to web development, we use HTML (user interface) and Javascript (application logic)
Thank you,
Erwin