How to Create an App for iOS, Android, or Windows Phone

Yes, it's true. The Web is dead. It's not that people aren't using The Internet anymore—they certainly are—it's just that users are moving away from the browser-based "Web" environment and into the mobile app-centric world.
Two quick illustrations: 1) Instagram has long been a true force in the social sphere, but it was years before it had a Web-based interface. Its entire model was mobile-based—the website was secondary. 2) Over the past years, we here at PCMag have even found it increasingly difficult to bring together our annual Top 100 Websites feature. Back in the early 'aughts, it was a sinch. Today, it's a stretch. (Good thing we have so many other great app roundups).
The moral: the Web is indeed dead, long live mobile. That also means all those hours you spent honing your HTML and CSS skills were for naught. It's all about being able to create a functional and engaging mobile app.
But that doesn't (necessarily) mean you have to take an app-programming class. Like the early days of the Web, several tools have risen that will allow anyone to create a product with little-to-no programming language. (But if you have the coding skills, it will give you the ability to make a truly unique thing).
These third-party services will even handle the process of submitting your app to the various stores (e.g. Google Play, the iOS App Store, or the Windows Phone app store). We'll get into some of those services below, but let's start with a very basic overview of each environment and how to break in all by yourself.
iOS
While Android is the larger of the ecosystem (worldwide), iOS is in the midst of a very impressive comeback. And if you want to make cash-money, iOS users have shown that they are far more willing to actually spend money on apps.
Before getting started here, you should also know that Apple maintains a far more stringent lock-down on its store than Google or Microsoft. You have to follow Cupertino's rules.
First, you will have to enroll in the iOS Developer Program, where memberships begins at $99/year (there are different levels for enterprise and education). If you aren't using a third-party development tool, you will have to download XCode—the official IDE (Integrated Development Environment) for the iOS and Mac environments—from the Mac app store. You don't necessarily need any programming knowledge to use Xcode, but it can help if you want to get into the nitty gritty.
The primary language iOS apps are written in is Objective-C programming language, a tutorial on that can be found here. Last year, Apple introduced the much less-techy Swift programming language to use alongside Objective-C, (which you can learn more about here).
Apple provides a basic tutorial of XCode usage here. Apple also offers an additional suite of tools that will help you test, distribute, and monetize your app.
After your app is developed and tested, you must submit it for review by Apple before it is included in the App Store. Apple's official stance is that "review times can vary for each app." If your app is rejected for some reason (Apple's long list of guidelines is here), you can submit an appeal to the "App Review Board." But ultimately, Cupertino always has the last say.
Android
To submit apps directly to Google Play, you will need to sign up for a Google Play Developer account, which will cost you a one-time fee of $25. Submitting apps to Google Play is a far less rigorous activity than for the App Store—once submitted, it should be available in a few hours.
If you want an overview, Google offers a nice step-by-step training guide for budding developers.
The first thing you'll need to do is download Android Studio. While there are other IDEs out there, Studio is the official Android one and is available for download on Windows, Mac, or Linux. If you want to design for Android, it might help to be acquainted with the Java programming language. It's not the only language that Android apps are built in, but it's the fundamental one. Android Studio gives you the opportunity to develop, test, and monetize your app. Once you have developed and tested your app, you will publish it using the Google Play Developer Console.
Windows Phone
Windows is by far the smallest mobile OS environment, but as it attempts to seamlessly bridge the Windows desktop, mobile, and tablet environments with Windows 10, it may become more robust. Time will tell if this strategy will be successful.
For an overview of the development process, Windows Phone has a nice development guide available here. First, you will need to sign up for a Windows Developer Account and pay a one-time $19 fee.
You will need to use Microsoft Visual Studio to develop the apps. Windows Phone—like regular Windows apps—are written in a variety of languages including C# or C++ with XAML, C++ with DirectX, and JavaScript with HTML/CSS.
If you want to test on your phone you will have to register it as a test device, though there are several Windows Phone emulators you can use as well. Once completed, you can just publish to the Windows Phone store.
Microsoft has also created a pared down developer suite in the form of the Windows App Studio, which allows anyone with a Microsoft account to create a cross-platform Windows app with just a few clicks. No programming knowlege required.