My advice would be to just paint and sketch as much as you can. Look at real world objects, look how light affects it. There are tutorials like, how to paint skin, how to compose a painting etc. out there from marc and other artists or watch experienced artists paint full paintings like WLOP or others.
With only those light-sources in your piece, everything should look a bit darker. important also is to set your Main light-sources in a piece, and not overkill with detail at first. Get all the basic light and darks, the general shape, dynamic and composition down, then go for colors, like your skin here is just grey instead of flesh colored.
So watch your surroundings and how experienced artists tackle painting and dont be afraid to study! Study, study, study ^^ you need to build up your visual library that you can pick and choose from to construct your painting. without those fundamentals it will look off and not realistic/ believable.
Hope this helps!
