Android, iOS and Windows devices all work a little differently and have their own stores for application downloads. When you're developing an app, you can't simply create one that works on every phone. Mobile app development is a much more complex process, so how can you decide which operating system to design your app for first?
Apple devices are significantly more popular than Android devices, with a much more loyal core market. This doesn't necessarily guarantee, however, that the situation won't change in future. Windows phones are the 'new kid' and are much less popular, though their uptake is slowly increasing.
Profit and competition
Apple's App Store is by far the most profitable application store at the moment. This can be a benefit, as it suggests that you'll make more money if you're monetising your app. Despite this, there are more applications on Android's Google Play. You'll want to strike a balance between profit opportunities and the amount of competition that you're dealing with, as well as bearing in mind that it's easier to have your app accepted by Google Play than it is to be accepted by the App Store.
What works for others might not necessarily work for you. Using Google Analytics, you can quickly view which operating systems your existing website visitors are using. If a majority of those showing an interest in your website are using Android phones then you might benefit from creating an app for those rather than one for iOS devices.
Windows phones are more typically used by business professionals, though younger smartphone users are beginning to show an interest in an attempt to be 'different'. Android and iOS devices, for the most part, have a similar target audience.
If your app is likely to be competing with another similar application then you'll want to research where that other application is published. You can choose to compete directly, or to provide your app where a similar one isn't already available.
Different app developers will charge different prices to develop for different operating systems. Ideally you'll produce an application for each different type of phone or handheld device, but otherwise you might want to consider your budget almost as much as you think about your audience.