You will need an original image, maybe a drawing you did, or a photograph. Or you can use public-domain stuff, which is what I do. No, you can't use copyrighted material. No. No.
Open an account at Cafe Press and open a shop. Yes, it's free. You can upgrade, of course, but if you're just starting out, there is no reason for that. Each shop, by the way, sells a particular image, so if you have a photo of a butterfly it would be your butterfly shop, if you have a drawing of your dog, it would be your dog shop, etc. I found this confusing at first, thinking that I could have a BradHallArt shop and sell various images from that. That's only for the advanced stuff. Keep it simple.
There is no need for me to explain the process of creating a shop on Cafe Press, they take you through the steps automatically and it works great. In fact, it's kind of fun. The tricky part is preparing the artwork. They have a page on how to prepare it correctly, what resolution, what size, etc., so be prepared to spend some time getting that right.
And there you go! Like so many things on the internet, it's worth a try. Your biggest investment is time, and you were just going to go look at Facebook, anyway, weren't you? When someone orders something from your page, Cafe Press creates the item, ships it, and makes a bunch of money, a small percentage of which is set aside for you. When your commission amounts to at least $25, they send a check to you.
Here are some of my shops if you want to take a look