Hello Sunil! Sorry for the late response.

Not sure my approach is the best way but my suggestion is:
Depending on your learning style, I prefer interactive learning, hence always lean towards video tutorials instead of books or articles. The downside is that video tutorials can be slower as you would follow the pace of the video, however, most video tutorials are easy to follow. There are many online courses now, Treehouse is really popular, Udacity is also popular. Those 2 are pretty good for starters.

Of course, if you already have programming experience, or better yet mobile development experience, you could probably pick up a book and dive into it, or just search around the web, or watch youtube tutorials (not all of them are step-by-step approach like Treehouse or Udacity).

Hope that helps! =]