How do I donate my Macintosh computer?

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

Your tag line says that you have a 21.5" iMac running macOS 10.15 (Catalina) – and you tell us that the Mac that you want to donate is over 10 years old. That suggests that you're thinking about donating a 21.5" iMac that cannot run anything later than Catalina – or at the very best, big Big Sur.

A machine that can't run applications available from the likes of Microsoft and Adobe.

Make sure that whoever you give the machine to understands this, and wants such an old computer for what it can run, e.g.

• Firefox
• LibreOffice
• Old 32-bit games (if they downgrade the OS to something earlier than Catalina)

and not for what it can't. If you just give an obsolete computer to someone who doesn't want an obsolete computer and who is immediately going to turn around and try to get rid of it themselves, you haven't done them any favors.

First properly prepare your Mac so your personal data is deleted. Then you may donate or recycle

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

Apple Trade In - Apple

Scroll down to the end of that page and read about how to recycle your Mac

Badah19_ wrote:

I want to buy new

See: Mac - Apple

Based solely on the information that you were using a 21.5" iMac, I suspect the following Macs would be of most interest to you:

• 24" M4 iMac. These have very nice 4.5K Retina displays that are much sharper than the display on your iMac. They have plain M4 processors that are low-end by current Mac standards, but that will blow the CPU and GPU in your old iMac out of the water.
• M4 and M4 Pro Mac minis. These are computers to which you add your own keyboard, mouse, and display(s). 27" 5K displays with the same sharpness as the 24" iMac display tend to be on the expensive side – even now that there are several third-party 27" 5K displays that are cheaper than the Apple one. But a lot of people use Mac minis with 27" – 32" 4K displays, and there are lots of 24" 1920x1080 displays if you prefer those.

In some countries, the online Apple Store has a Certified Refurbished section.

Refurbished Mac Deals - Apple

The selection there is very "catch of the day", and you can't custom-order a particular configuration, the way you can if you buy new. The discount is usually 15% off original list – which may or may not be a good deal depending on the particular model.

But right now, in the U.S. Certified Refurbished store, there are a bunch of M4 Mac minis, M4 Pro Mac minis, and 24" M4 iMacs – that is, to say, current models. Given the quality of Apple's refurbishment process, if you could find the particular configuration you wanted there, that would be an easy way to save some money.