No worries.
That model iMac officially supports running windows via Boot Camp (with Mac OS X installed) or in a virtual machine within Parallels or VMware Fusion. It doesn't support installing Windows directly to a blank drive, as there are no Mac hardware drivers built into Windows.
In addition, due to its age, the latest version of Mac OS X it will run is 10.11.6 El Capitan, and it won't support the latest versions of Windows in Boot Camp:
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Install Windows 7 on your Mac using Boot Camp
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Use Windows 8.1 on your Mac with Boot Camp
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Install Windows 10 on your Mac with Boot Camp Assistant
You may be able to run newer versions of Windows in Parallels or VMware Fusion, but you would need to find an older version of those as well, since the current versions will to run on your iMac and/or El Capitan.
There may be workarounds out there to install Windows directly on that model iMac, but it isn't something I have done or recommend.
C