Hi Eliana,
Probably one of the sites I would recommend would be Fotolia (now Adobe Stock). They are relatively easy to start with, and have a large client base. The key to making money with stock is to upload a lot, and to upload frequently (something I am not good at), but even if you have a portfolio of 20-30 images uploaded, you should start to see some money coming in from your images. I’ve tried 6-7 different stock sites, and Fotolia is the place I’ve seen the most purchase/search activity.
You can sign up here:
https://www.fotolia.com/Info/Contributors
A couple other places to consider would be:
1. IstockPhoto (they have a large client base, although since their merge with GettyImages, they’re a little harder to sign up with)
2. 123RF (easy to start with, not the best market)
3. BigStock (easy to start with, not the best market)
4. Dreamstime (easy to start with, not the best market)
Hope that helps!
Greg