iStock is a good one to start with, as is Alamy. You need to shoot a great deal to earn anything from stock beyond a few quid here and there. It's something you build over time. The stock market is in a bit of a decline at the moment though... too much royalty free stuff and too many people doing their own in house stuff because they think it's easy.
Treat stock as a side-line or even a hobby as you build up a folio of stuff. Just read the small print carefully.. some ask for exclusivity with leaves you little option professionally elsewhere if all your work is tied up legally.