How do I create a bootable macOS X installer usb from a windows computer.

I have an iMac that my brother gave to me that isn’t functioning properly. The first issue was when turning it on the Apple logo would show and then after a few moments the prohibitory sign would show. Tried going into recovery mode but that brought me to a firmware lock screen which my brother didn’t know the password to and apparently has no idea how he locked it but I’ve fixed that by removing the ram and doing a PRAM reset. Now I’m back to the Apple logo and then prohibitory sign when doing command-r. Turned it off then back on this time choosing the option key to bring up the startup manager which did work and it showed me two options, one being Macintosh HD and the other just said 10.9.2 I believe. Clicking on the Macintosh HD takes me back out to the Apple logo and then prohibitory sign while the other option brings me to the Apple logo and a spinning loading circle underneath but never progresses to anything else. I tried booting in safe mode which did not work and I do not believe this model has internet recovery capabilities, it’s a late 2009 10,1 model. From what I’ve researched a bootable installer usb may be the solution but I’m not sure which macOS install version I’d need or how to create this usb file from a windows computer. I don’t know a whole lot about Macs since I’ve always used PCs but it would be really awesome if I could get this one running properly.