1) Know your brushes and how to use them...the best way to get to know your brushes is to practice with them to see what you can do with each one. The more you get to know your tools the better results you will get with your art.
I have been asked what Kinds of brushes should I use for watercolor? I say there are a couple to keep in your tool box...soft brushes made for watercolor---a mop, #6 # 12 round and a rigger, but all are a matter of preference.(they do not need to be expensive) Me personally, I mostly use the round #12 because you can use it for a variety of strokes. It has a point and full round body.
2) understand how to start your paint, mix your color, and how much water to use. This can be a little tricky but with a little practice it will become second nature. I found if you start by spraying your color pallet with a small misting bottle filled with water; this will moisten the paints on your pallet and makes it easier to move pigment to your mixing well.
3) With watercolor you will want to build your paint for light to dark. What I mean is you will want to paint a layer -let dry then add more layers of watercolor each layer making areas darker which adds values (depth) in your painting.
4) Know the differences in wet on wet & wet on dry. Wet on wet is where you wet the paper or area before adding or dropping the paint onto the paper were as wet on dry is where you brush the watercolor paint onto dry paper. Also, understanding that watercolor paint will only travel on its on where the paper is wet or damp. This is what makes watercolor different from all other medium and is way watercolor is considered magical.
5) Paint to water ratio/value...what this means--the more water you add to a pigment the lighter the value 1:5 ratio and the less water you use in the pigment will make the value darker 5:1.
If you get the hang of these few techniques you will be on your way to a successful watercolor painting. Try them and remember it is never to late to create. Have Fun!!