How to reinstall Mac OS Snow Leopard from Grub

I tried install Ubuntu on the internal hard drive of my iMac and now want my iMac running Snow Leopard again

I can acquire a Snow Leopard ISO and burn it on to a CD through using my laptop. I don't know if that helps but I have that option. Grub does come up when I boot the Mac initially. But then I don't know where to go from there. Would it be wise of me to install the Windows 10 first that I have or should I just install the Mac image that I have?

Solution 1:

Either the Mac came with installation media which you can boot on (may need to press C or D while booting), or it has a rescue partition built in you can boot from using Command-R while booting (see https://support.apple.com/en-us/HT201314)

Then proceed to use Disk Utility to completely wipe the drive, and then install MacOS/OS X.

Full details on https://support.apple.com/da-dk/HT204904