resetting an older iMac

What year model is the iMac?

see > Identify your iMac model - Apple Support

If you want to reset it for resale, donating or recycing?

see > What to do before you sell, give away, or trade in your Mac - Apple Support

If you just want to erase and reinstall the macOS?

see > How to reinstall macOS - Apple Support

You can use the following instructions to completely reset a Mac to factory settings. Note that step only applies to newer Macs and that you will use the other steps instead. What to do before you sell, give away, or trade in your Mac - Apple Support