I have a 15 year old iMac. I want to erase it before dropping it off at an apple store. How do I do that

Follow the steps in What to do before you sell, give away, or trade in your Mac

Startup from the gray Install Disc 1 that came with that iMac. Go up on the menu bar to Utilities > Disk Utility. Open Disk Utility and Erase the Macintosh HD.

A second option if it is running a later macOS is to see if it will startup in macOS Recovery. Then open Disk Utility and Erase the Macintosh HD.

If neither of those work, call ahead and check to see if the Apple Store would be willing to do a Erase before recycling it.

Another option for a machine that old is to remove the HD which is pretty simple assuming you have the tools. Here is a link to a video if you are interested, 15 years old would make it a 2007 so here are videos:

20" 2007 iMac How to Install a Hard Drive in the iMac 20-Inch (2007 - (macsales.com)

24" 2007 iMac How to Install a Hard Drive into a 24-Inch Apple iMac (macsales.com)

+1 that’s what I did with a couple of 2006 iMac’s a few years back.