Watercolor is a quick way of painting. I don't think it is necessary to go on building a watercolor into a complete, tonal painting. For that use opaque watercolor or oil.
The charm of a transparent watercolor is its delicacy and simplicity. So when trying out a simple pictorial image, lightly draw in your subject with a lead pencil. Don't put in too much detail or any tone at all. Then by quick, simple washes lay in the appropriate colors, letting each color dry first before putting on the second. If what you were after doesn't quite click, don't try overworking it. Put it on one side and try another and another. The more quick sketches you do the better you will understand watercolor.
Always have plenty of clean water handy. At least two jars of water should be on your table. Outside, this is not convenient, but do try. If you find that your success with handling pure washes does not come easily, try drawing out your picture first in pencil or pen and ink and then tint the completed drawing with simple washes, gradually drawing less and less so that finally you are not drawing at all, only painting. Or, conversely, block in your drawing with large washes of color and then draw on top of the washes when they are dry. I find this last method very suitable for outdoor work.
If you have drawn a great deal, the handling of washes will not worry you unduly. It is much better to have done a great deal of drawing before attempting pure watercolor. But if you haven't, then I can best recommend opaque watercolor.
Avoid bright greens, Indian red, black (except for a monochromatic painting). Don't try and touch up a pure watercolor with white. It ruins the subtlety and quality of pure watercolor. Have a clean rag handy for sopping up too watery washes. Remember that your whitest whites are the untouched paper itself. If you can't soak out a white, scrape it lightly with a razor blade when dry. You can sometimes lighten a color by using clean water with your brush, by painting the part you want lighter and then blotting it with clean blotting paper. Don't use too many colors to begin with. Do your first painting in one color alone, say umber or black, in other words a monochromatic sketch. Then try another with umber and black. Then another with umber, black and blue. Lastly try a sketch with umber, blue and red, leaving out the black altogether. After this do a sketch with all the colors you have in your box, and remember, small pictures are easier to handle than larger ones.
Watercolors, unless strengthened by strong drawing in another medium or unless they are painted very brightly, don't frame up awfully well, because of the constitution of the paint, and tend to fade in strong light, even under glass. They are best kept in a folio and looked at occasionally, as they haven't the tonal range to carry across a room. Because of this I have no hesitation in recommending a beginner to go straight on to the opaque watercolor media first as they are much easier to handle and are in some respects very similar to oil.