To get started with the development of windows phone 8.1 apps , the developers should have the Windows Phone 8.1 SDK installed on their system .
You can download Visual Studio Express 2013 for Windows , a free tool that lets the developers to create Windows Phone and Windows Store apps, including universal Windows apps targeting all the Windows devices. The tools include the necessary Windows and Windows Phone SDKs and tools necessary for creating , testing and debugging the app. If you are on Visual Studio 2013 Professional , then you can just install Visual Studio 2013 Update 2 or higher which installs the SDK for developing Windows Phone 8.1 and Universal Apps.
Below are some of the softwares to download and install for developing Windows Phone 8.1 Apps