First you must choose the figure with which to paint. For beginning painters this can be a hazardous task. If you want to become a serious painter you must take things in steps. Do not purchase figures that are above your skill level. Start off with simple figures that will train you to paint one step at a time. I recommend starting with a figure that has large flat areas. Get a few figures under your belt before starting on those with flowing robes and multiple layers of clothing. This will teach you to paint in thin coats and obtain an even surface free from brush strokes. Selecting figures that have open and free flowing movement are best to learn shading and highlighting do not try to paint those with covered areas such as arms in front of their chests. These areas are difficult to get paint into, they will invariably look muddy and untidy.
Prepare the figure to give you the best possible advantage when painting. File or trim all seams and flashing. Using good tools and taking your time will pay off when it comes to painting. You have enough to worry about while painting let alone trying to figure out ways around a flaw on the figure. Use a round file whenever working on body parts, flat files are for swords, axes, belts, and things generally flat in life. Affix the figure to a painting base or to it's permanent base. You do not want to touch the figure with your hands once you start painting.
Undercoating or priming is one of the most abused steps of all. Personally, I use an airbrush so that I can control the amount of the paint. I have tried many spray primers and I find that the best are flats used for automotive priming. Be careful to not get any that are oil based, there are a few out there, your paint will not adhere as well and leave a glossy finish which is hard to work with. The choice of black, white, or gray are up to you and the look you want to achieve. I mainly use white for show and fantasy pieces, black for 15mm and regular infantry for wargaming, and gray for 54mm and larger. Expirement with what works best for you and your painting style.
I mix my basecoats with a 1:1 mixture of paint to thinner. I use a mix of Matte Medium and purified water. You can substitute Gloss Medium or mix Gloss and Matte to get the desired effects needed. I use this mix so that my paint stays where I want it to and to keep the paint on the edges of the figures. If you notice that paint easily rubs off of the edges you are probably only mixing water into the paint. I only paint the one area at a time, all flesh, all armor, all weapons etc. until I am done.
It is important to note that because when we mention shadows or washes, we do not just slop a lot of paint on the figure. It is important to always draw your bursh out on a cloth before painting. Paint control is important, you can ruin a figure if you do not pay attention to how much paint is in your brush. Shadows are painted with a mix of 3:1 paint to thinner. I do 2 shadow steps, the first is the darkest of the dark, the second I save until the figure has it's last highlight done. This last coat is mixed 10:1 paint to thinner. I do this to draw the entire figure together and cut down on some of the highlights. It is barely perceptible but unifies all of the paint with the same look. Alway paint shadows from the bottom up with the brush at a slight upward angle. Try to notice when painting where the paint concentrates when you lift off from the figure. It is always at your last brush stroke, in this case downward which is where we want the shadows to hang out.