I wish to dispose of an old iMac

You can prepare the iMac as Chattanoogan described and then take it to the nearest Apple store for recycling

If you follow Rudegar's suggestion about trade in and find it has some value do the above and take it into the nearest Apple store to get an Apple gift card for the value stated.

you can do it using their trade-in Apple Trade In - Apple

See also this article:

https://support.apple.com/en-us/HT201065