Then start simple: just spray this tank olive drab, aiming for an even coverage. Don’t bother with things like pre-shading, post-shading, in-the-middle-shading or whatever yet, but instead focus on getting a feel for the
airbrush, how it works and how it responds when you do things like pull back the trigger some more or a bit less, or vary the air pressure.
Shading and highlighting also doesn’t need to be complicated: take a colour of paint that’s darker than what’s on your model (for an olive drab tank, a dark brown works well), take some of it onto a palette and thin it so it flows like water to create a
wash, and brush that all over the model. If it covers too well, brush more water over it before it dries. You’ll probably have to mix up some more of the wash every now and then, as it’ll be easier to make just a little bit at a time than enough for the whole model in one go.
Then let the wash dry, but take care that you wick away drops that form, tide lines (where a whole line of the wash descends down the side of the model), etc. before they can dry. If you don’t, they will remain visible forever (unless you touch them up, of course).
Once dry, take a paint that’s lighter than the base colour, dip a fairly large brush into it, wipe most of the paint off on a rag or a piece of tissue, and
drybrush the model. The paint will stick on the higher parts, highlighting the model. Do note that not all paints are very suitable for drybrushing —
Tamiya acrylics are fairly difficult to do this with, especially once the paint in a given bottle starts getting older. Things like Vallejo
Model Colour,
Revell Aqua Color, or any enamel paint, though, usually drybrush quite well.
As for varnishing, I don’t usually at all. A good example of why is actually
the Churchill tank that SteveT just finished:
I dread this sort of thing happening every time I spray varnish onto a model, and though some people swear by it, I most certainly don’t
Generally speaking, I only see a need to varnish wargames models, not ones that won’t be handled. The main exception is if I need the finish to change, such as if the model is currently matt but needs to be glossy. And even then I don’t like it due to the risk of something like this happening, as well as for the way the model will become darker when you varnish it.