the way i learned was from my life drawing classes
first the most important thing is to draw the general outline of the body
meaning the spine, the shoulder, the hip, and then arms and legs
these are really simple lines just to make sure you have the right pose drawn, and that the anatomy looks close enough to be correct
gesture drawings are what they are called
thats what you always start with and then build on it
we used to have practices doing gesture drawings which last only 1 minutes per drawing
this tutorial is close to what i used to do
http://www.ndoylefineart.com/gesture3.htmladdition to that we learned to look at general shapes and sides of a body
for example the head in general can be looked at as a simple rectangular box. the top of the head is one side of the box, face is front side, cheeks are the left and right side, back of the head is back of course, and bottom is the underside.
building on that shape you can easily define more and more of a human face. the important concept was to clearly define the sides of the face so you know where the shadowing starts at which edges.
The realistic rule
for eyes, nose, and mouth placement, it is usually safe to say the eye is half way of the entire head(top to chin), then nose is half of the space between the eye to chin, and mouth is half of nose to chin
However, this rule doesn't apply nicely to anime style drawing since their eyes are usually huge and the mouth tends to be really huge or small and even drops off the face at times... its really tricky, its more of a style approach