Shutterstock is the best-known site for selling photos. It's fairly straightfoward to use, and has a good reputation.
But don't expect to make any serious money from it. Unless you're a particularly skilled photographer with a large collection of photos to sell, the chances are you'll only make pennies. That's not just true of Shutterstock, but of all the similar sites.
But don't let that stop you giving it a try.