With roughly 1.6 billion people around the world now employing a smartphone, the mobile app market has never been more exciting or lucrative, with Android and iOS dominating the smartphone operating systems.

You will need to decide exactly which sort of smartphone user you would like to focus on first if you don’t want to create both at an equivalent time.

As they're two very different mobile platforms that need two very different types of language (Android apps are predominantly wiped out Java, while iOS ones are mostly wiped out Objective C or Swift) and native UI / UX design.
For total penetration, you ultimately want to develop for both iOS and Android.
