If you’d like to create a Windows Store App but don’t have access to a Windows 8 computer, you can build a virtual Windows Store app development environment (for free) using taking the following steps.
- Sign up for Windows Azure using the free 90-Day Trial Offer
- Create a virtual Windows Server 2012 sever on Windows Azure
- Download and install Windows 8 development tools
If you want the steps in details check this blog:
Know the steps in details from here