Need some advice about app developing?

[Paul]

Depends on the platform for which you want to create.

For Android app development, Core Java and XML are required.

iPhone apps are developed on Objective C and also you need Mac OS for developing iPhone apps.

Windows apps are developed on Visual Basic C#.

If you are passionate about learning it then start with Android app development. First, learn Java for it and then dive Android app development.

Try the following resources for learning it-

http://www.tutorialspoint.com/java/index.htm
http://docs.oracle.com/javase/tutorial/
http://developer.android.com/training/index.html

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By the way, HTML, CSS and JavaScript are web development languages. It is possible that you can create apps with it but only the simple not the complex ones.