Welcome - hope you can find something useful here as you consider shooting stock.
This site isn't geared to tutorials - but I assume you've already found other places on the web for that. I also assume you've had a browse through what's currently available on the top tier stock sites - that'll give you an idea of what you need to be able to do if you want to contribute stock (and focus on the D40, not the Coolpix as you work towards stock images. Lighting is a subject lots of people struggle with at first, and if you haven't looked at the
strobist blog, I'd highly recommend it - their Lighting 101 is a good place to start