They are totally three OS is used to develop windows phone application:
1.windows Phone OS 7.0(Older Version)
2.windows Phone OS 8.0
3.Windows Phone OS 8.1(Current Version)
You start to develop application in windows phone 8.1 is compatibility for all other OS(7.0 && 8.0) so if you are beginners try to start develop in Windows Phone OS 8.1.
SYSTEM REQUIREMENTS:
1. Microsoft Visual Studio 2013 with update 2
2. Windows Phone Emulator 8.1
3. System OS Windows 8.1(Pro)(avoid Single Language OS Because if you need to run the project VIA Emulator means you need Hyper V)
all the best happy coding................