Let's start with what item you need for your camera as of now. Do you need a flash, lens, camera bag, battery pack, etc? Also, is it only from Canon website of can it be any store. Also, what is your budget range as it will give us an idea here to look into.
For now, you have Canon 20D which is only take photo only so, no video in that camera. I guess, based in the limited info, you're doing portrait photography which mean either you need lens for it and maybe flash. If flash, if you want to keep it cheap, go to Yongnuo yn560 III or IV (get the IV if you can get it cheap as it have a build in transmitter or receiver). It's a manual flash which give you the basic necessary stuff you need for this flash and it's actually decent in power and price.