The thing is to create compositions in values, to organize the space without leaving out empty space. For that; avoid drawing only outlines (contours) instead put in zones of values, treating the whole.
Making many of these will help you to find the most interesting composition for any subject you want to paint.
So, before starting, when you're in front of your subject; do swatches! Don't jump right into it, anyhow, you might loose a lot of time, struggling with a "non working composition". Without knowing why your painting doesn't seem to function. It will help you to avoid painting something not worth it, as uninteresting when badly organized; maybe to central or symmetric, to flat or not knowing where to put you point of interest and not making the eye travel around in the painting later on.
It also warms you up and puts you in the right state for painting.
Some advices : You can use pencils from HB and softer (HB to 9B), if you only use a HB pencil it might not be soft enough to mark you darks.
For the watercolor swatches, chose you compositions out of your drawings and try to paint freely, to create a spontaneous feeling. Avoid "fiddling" with details, the small size of the swatches are supposed to make you go for the essential spaces, not the details. We always tend to pay too much attention to details in the beginning.
You can use photographic material as a base if you lack imagination. But try not to simply copy, try making it better or to develop it, to make it your own idea.
A remainder : Details are not what make you like a painting, composition is! A composition is the organization of the space, in values (when painting). When you learn to treat values, and translate what you see into values, you will understand a whole lot better how to paint in color.
Values are the range of "grey tones" between the two extremes of white to black, you should use them including white and black. Avoid making composition with equal amount of white grey and black (1/3 of each). It's much interesting having "a touch" (or a little more) of a value and having a dominant.