Q: How do i put windows 10 in bootcamp

The following may help: Use Windows 10 on your Mac with Boot Camp - Apple Support

you can install it in a virtual machine like virtual box or get an older version supported by your mac and then update that version to windows10 but there may not be driver support for your older macs hardware

What year/model is your iMac? Is it listed in link that FoxFifth has provided?

If you have a valid win 7:

install win 7, install the bootcamp supplementary drivers, make a complete "patchday" to win 7, that means many many updates, reboots, really many reboots, and updates again until no more updates are presented.

Then make a gwx update to win 10. Maybee you need the microsoft media creations tool to download the win 10 update.

For my old mac Pro 2010 / 5.1 this labour intensive and awkward procedure had successed to install win 10 on the "not compatible" machine. Works perfect.

greetings from germany

Chris