What mobile OS should I choose to start developing apps? iOS, Android...?

Answer 1

Android is cheaper in terms of hardware and publishing. For iOS you need an MacOS-compatible system, which are obvisouly Macs. Programming for android requires just a simple pc, with Windows, Linux, Solaris, BSD, MacOS, ...
When you want to publish your apps, the AppStore costs you 50$, where PlayStore fines 25$.
Also, Java is an allround language used almost everywhere. Embedded, Games, Printers, TVs...
Swift is more an applish thing. It hasn't any important meaning outside of the Apple Universe.

Answer 2

Well it depends who your user target is. If your target users are using Android devices, you could consider that as your main OS. But now imho the choice of Android vs iOS is hard because the Mobile market is so fragmented. Users can also switch from Android to iPhone which could be a problem if you would focus on 1 OS only. I would suggest for you to look into Cross Platform Mobile development tools/frameworks. You’ll be writing in 1 language and dealing with 1 codebase for multiple platforms. Here is a short list of interesting frameworks:
- Appcelerator Titanium (Based on Javascript, compiles to Native)
- Xamarin (Based on .NET, compiles to Native)
- NativeScript
- ReactNative
- Phonegap / Cordova (more a mobile web approach with device APi access)

You could go for Native Android development and Native iOS development, but then you’ll be dealing with 2 codebases = 2 x design, 2 x tests, 2 x bugfixing, ...