Quite a bunch of options, some include that you have got a Mac (which is the free version; unless you've given all of the money to Apple for buying the products), and you install the Xcode IDE and do it all.
Secondly, you can use other third-party frameworks, to build iOS applications without using Mac for installing the IDE. Such as, now you can use
Xamarin[
^] to write applications in C# and then compile them into native apps for cross-platforms (including iOS). But that also requires you to have a license before you can even compile the application.
Leave that, come to another stage.
Visual Studio 2015[
^] comes with a lot of features and support for the cross-platform programming in C++, and you can compile the source code into different platforms all from your Visual Studio IDE. This it enables you to use the same source code, and then compile and create different packages of the same application.
Don't like C++? Come to another next stage, there is another framework known as
Apache Cordova[
^], it enables you to write the applications in HTML5, CSS3 and then control the program's actual logic using the JavaScript (or its libraries; jQuery etc). You can similarly compile and build the packages from the same source code. Enough of these package, aren't they?
One thing you need to consider is that Mac is required for debugging the iOS apps. There is no way you can use any emulator of iOS to test apps. Visual Studio 2015 comes shipped with Android emulator, Windows Phone emulator, but it needs that you connect to a Mac before you can actual test the applications. That is why, my last point is that if you're going to create applications you're (
one way or another) going to require a Mac.