You have 2 main types of sites, 1 print on demand (pod) and 2 stock. I have already written about pods so for this post I will discuss stock sites. Stock sites are used by companies looking for images and it is cheaper to buy an image than pay someone to go take that image. What their willing to pay will depend on the size of the company and what the image is to be used for. If it was world wide advertising campaign you could be onto a good earner but if its just for a few hundred pamphlets their budget will be a lot less. The thing about photographs is they can be used in many different ways for example a photograph of a glass of wine could be used by a bistro, wine bar, nhs (drink aware), wine maker, wine retailer, vine yard, dentist (teeth staining), for cooking, well you get the idea, landscapes are popular for tourism, a man smartly dressed could be used a hundred different ways.
So now you want to sell your images, you need to select a stock site but be careful the payouts vary, the rights vary, the requested file types vary so a little research into which site you pick but I would recommend you look at Alamy, Shutterstock and Istockphoto to begin with.