Hi I have a desktop apple computer that is probably 12 years old. I want to recycle it, how do I delete everything from it first?

You have two options:

• Upgrade the iMac to OS X El Capitan, then macOS High Sierra. This will allow you to use the Recovery Mode to erase and reinstall macOS. Although, if you upgrade, you might be able to extend this Mac's life since you have such a newer version of macOS installed and it is much more usable. For this path, first upgrade to OS X El Capitan, then macOS High Sierra: How to get old versions of macOS - Apple Support.
• Use the Mac OS X disks to erase and reinstall OS X. If you have these disks, you can erase now without needing to Upgrade.

Let me know which option you'd like to continue with.

Jack

OS X El Capitan will download a file called "Install OS X El Capitan". Click on that file and install the Installer App. Now, open the App and follow the prompts to install. After installation, you will be on OS X 10.11.

Then, you can get macOS High Sierra from the App Store and repeat the process.

Jack

You don't update like that.

Get OS X El Capitan here: How to get old versions of macOS - Apple Support. After you upgrade to OS X El Capitan, upgrade to macOS High Sierra using that same link.

Jack

Welcome to the Apple Support Communities.

Please go to the Apple menu (top left) > About This Mac. What is the year and current macOS version?

Jack

Do you have the original Mac OS X Install disks?

Jack

And it doesn’t allow me to upgrade.

Why not? What error does it say?

Jack

First, you need OS X El Capitan.

Oh, OK. After you've run the Installer and installed OS X El Capitan, do the same for macOS High Sierra.

Jack