If you start developing for Android or iOS?
IOS and Android are large platforms, and none of them is perfect. The platform you choose will depend on preference and the type of market you want to target. For example, my first native app for iOS was just because I had an iOS device at the time. I eventually moved to Android for two reasons. First, because a client wanted me to build an application in Android and secondly because the Android market is larger than the iOS market. 
My recommendation is just pick the one you like the most and start developing for it. Again, no platform is perfect, and you will find the jobs and resources for the platforms.
What are good ways of learning?
Whether you are developing applications for Android or iOS, the best way to learn how the development is simply the application coding and practice. Reading is essential, and watch tutorials as well, but they are useless if you do write code and create applications. My recommendations to accelerate the learning process are:
Create applications that you would like for yourself: They should not be complicated, beautiful or perfect. The goal is for you to practice and become familiar with the SDKs. As your skills improve, you will naturally add more complexity to make better UX interface, and perhaps earn money with your applications.
Try recreating existing applications: There are many YouTube channels dedicated to recreating existing applications. The good thing about these channels is that you get to implement the entire application and learn how others implement certain functions.
Create prototypes: Let the word you want to learn how to use a new library, but you do not have a specific project or idea where the implement. Then simply create a small prototype which aims to test and try the library you want to know more about. Save the prototype on your GitHub account and use it as a reference for any future project.
How to stay updated
The beauty of the computer world is that technology is advancing very rapidly and what is relevant a year, can become obsolete following. This is also a curse for stay updated can feel like a job. I use mainly the following three sources to stay current with the latest technology and mobile development trends:
Official documentation: Follow Google and the Apple official website, as well as the Android YouTube channel to stay updated with the latest trends, tips and news releases.
Forums and Blogs: There are tons of forums and blogs out there. In fact, it is impossible to follow them all because there are just too many. In the medium and Reddit are my favorite sources. In the case means I am the people who are involved in the Android community and regularly post articles. For Reddit, I r / AndroidDev and r / Android to stay updated.
Finding a job: To find out what organizations are looking to mobile applications developers, I recommend you to read the description of mobile development jobs. You will find that companies of technology and expertise are looking to mobile developers. The information will guide you on what to focus your studies, what skills to develop, and what libraries to learn.
Now more than ever, there is a significant need for mobile developers. There are several ways to develop mobile applications; however, I recommend starting my native development. Android and iOS are the two major platforms. Just choose the one you like the most and start developing for it. The best way to learn is to create simple applications for yourself and try to recreate existing applications. Finally, it is essential to stay updated with the latest news and trends. To do this, follow your favorite blogs, authors and reading job postings to stay current with the technologies and skills companies are seeking.
Do not forget about yourself and stay updated for future articles.