Design

1. Choose one major platform
The major platforms in mobile app development world are Android, iOS, and Windows. There are others too like Symbian and Blackberry RIM, but you already know not to go for it.
So, either you can go for all the three major platforms, or you can choose one.
Even though Window 10 apps are springing up, it's still not the most lucrative to areas, to begin with.
Thus, the first stepping stone for your journey to choose will be either to opt for iOS apps or Android apps.
Once you have control on one, you can expand your horizons to include other app development technologies too.
2. Get required training and education
Getting a formal training and education in software development processes is indeed required.
A degree in computer science and software engineering is a good foundation because it helps you get a grasp on key concepts of data structuring, designing, and programming.
But even if you have passed the graduation age and still struggling where to begin with, you can opt for online courses. There are several web platforms which offer online courses specifically in app development.
You can even opt for free app development courses at various website. For example, Coursera, one of the leading providers of open online courses where you can learn how to develop a mobile app.
3. Master these three areas
Like any other discipline, app development requires you to take a strong hold on fundamentals. Here, the three core concepts are mobile user interface design, programming, and business expertise.
Mobile user interface design:
It all starts with the design. There are beginners who complain when asked to learn the intricacies of the interface. But that's the core of an app. Being able to code an interface sets a launch pad for your career growth trajectory.
Programming:
It's not the programming language which matters the most. It's your ability to make logic and translate those algorithms into efficient codes.
You should have knowledge of programming if you want to develop a mobile app.
However, you still need to learn a few languages to make it work.
1. Classic object-oriented programming language: Java, C+, Objective C. For iOS, Apples own programming language Swift is also being widely accepted by app development companies.
2. Front end: HTML5, CSS, and JavaScript
Business expertise:
Missing a few fundamentals can work. They can be strengthened. But you should always try to understand the business cases of an app.