what will be the first step to change my Mac OS to windows?

bavneet275 wrote:

what will be the first step to change my Mac OS to windows?

The first step would be to determine if Boot Camp is supported on your model/year computer. Next, follow these instructions:

Get started with Boot Camp Assistant on Mac - Apple Support

You could also install Parallels and then install Windows in that app.

bavneet275 wrote:

what will be the first step to change my Mac OS to windows?

On the off chance you meant how to completely replace macOS with Windows, you can find numerous sources online by doing a basic search.