How do I install macOS High Sierra on my iMac?

What is the model year of your iMac? The model year of an iMac is available at Apple menu > About This Mac.

A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. 

A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later. 

A mid 2014 and late 2014 iMac can be upgraded to the latest version of Big Sur macOS 11 but no later. 

A late 2015 or mid 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later. 

A 2017 iMac can be upgraded to the latest version of Ventura macOS 13 but no later. 

For all of the above, see instructions in: How to download and install macOS - Apple Support

A 2019 or later iMac can be upgraded to the current version of Sonoma macOS 14. See: How to upgrade to macOS Sonoma - Apple Support

Your question is somewhat ambiguous but to keep the answer short, and if your Mac is eligible, one option is to use Internet Recovery with this workaround (I wish Apple fixes this):

https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/