CJ is right, you can let the figure beat you if you worry about messing up. You'll do fine.
Keep in mind that no matter what happens, its a step forward. We always learn from our mistakes and sometimes it takes a major mistake to learn the most. There are no steps backwards, sideways maybe but never backwards. Make an error? you learn that you can't do that and you also learn how to repair mistakes and as your skill set grows, so does your ability.
Take chances, try new things and there is nothing (short of dropping in to the oven and melting it) that will "ruin" a figure by painting. Its one of the reasons they make thinner/oven cleaner/simple green and any other number of paint stripping techniques. Its also not unheard of to go back after you improve your skills and time passes, to strip and repaint figures you weren't entirely happy with.
If you are really the cautious type, and are worried about results, take your time and do something in small steps. Lets take the shading techniques. Go with several very thin layers rather than trying to over do it with 1 or 2 heavier. Its always easier to add than remove. Same with highlights. Your first highlight layer (not unlike a good drybrushing application in modeling) may be almost invisible to the eye. The second layer adding just a hint more. This allows you to control your highlighting/shading and you can build up your layers (think of a topographical map if you are using acrylics) till you are happy with the result. The only thing it costs is patience and time. Too many errors come from rushing a project and wanting to get it all done too soon. A single coat of paint for the base coat should be 2 or 3 coats. Allowing each coat to dry before adding the next. You can cheat a little and use a hair dryer to speed the process up a bit but the principles are the same. Same as if you are using oils. An acrylic base coat with oils used to highlight and shadow must not only dry but cure for the best result. This takes time (24 hours) and will provide a better result, a more stable work surface and not suffer from the thinners, brushing and work of subsequent steps to achieve the finish.
If you're unsure, set a goal for your work session. Tonight I'm going to apply my first highlight layer. Then walk away from it for a bit. (hour, day, week) and take a look at it with fresh eyes. You should see a slight difference if you started right but almost imperceptable. The next layer can sometimes be the same paint ratio as it will add a second layer of paint over your first. Leaving an area covered only by one layer and doubling up the rest. This is more pigment, making it more opaque. Add some more lightening pigment and paint your 3rd layer. Again, you are adding a layer and covering paint so it should be lighter when it dries. The more layers, the smoother the transition to your high light. If the contrasts are too high, mix a glaze of your base color and overpaint. This should smooth out some of the sharper contrasts. Since newer paints are finer pigments (vallejo, andrea, reaper) you can really build up the layers because the paint and emulsion are so much thinner and not obscure detail.
Good luck
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"