The trick is to be patient. Well rendered paintings in any medium is a tedious task
Get a good, small kolinsky sable for dabbling and small rendering strokes. Get a larger one to block in the lightest tones (you must paint from light to dark in watercolor). Get a scrap piece of cheap watercolor paper to test the color and value on your brush before putting it on the painting. If you make a mistake, wet the area with the brush and wipe it off with a paper towel (you will do this often as a beginner). Keep mixing the paint and testing it on the scrap piece of cheap watercolor paper until the value and the color is correct, then put it on the painting.
If you can't even draw or render well in graphite, it is going to be a lot harder trying to render in watercolor because you will be trying to tackle a difficult medium and at the same time trying to learn how to draw and render..