I've learned a little by trial and error, trying things. One thing is that the principles are the same, whether you're painting a StuG or a French hussar 
That is, I'll use a darker shade of the field color when coloring panel lines, than just to put black in it. If the field is dark mustard yellow, I might use a medium brown for that panel line.
I look at shading on a face the same way. For Caucasian faces, for example, I like to use Andrea Beige, or Vallejo's light skin colors, for a base. Then, I'll take a burnt siena and add a tiny drop to some of that base color, mix it thoroughly, and then apply that to the folds on the face--the bags under the eyes, for example, or in the lines from the corners of the mouth up the sides of the nose. Just like doing a panel line.
And then, if I'm using acrylics for the base, I might use acrylics for the shading, in which case I thin the shade color and just keep applying it, building up the color a little at a time, till it looks right to my eye. Or, I might use oils for shading over the acrylic base. In that case, I'll mix a shade color, again, looking at my base and then picturing it a little darker, even to the point of looking in the mirror to help visualize it, and then I apply the oils to the folds. Now, the beauty part about using the oils is their slow drying time, so I can wipe away the excess, leaving the color in the folds, and I can also use the brush to remove the color, brushing towards the lighter areas, and creating a blended transition from the highlight to the shadow.
Generally, with Caucasian complexions, I find a medium brown or tan to be very useful to use as the basis of the shadow. For black complexions, I tend to work in the other direction, that is, applying a darker base color and then using a lighter shade of that color and hitting the highlights, like the cheekbones, the bridge of the nose, the forehead, chin.
I hope that sounds clear. In any case, please do keep painting! I think you'll find it gets less intimidating and you develop a technique, the more you do it.