I usually start with a quick, stick-figure like sketch of the pose - a circle for the head, and then a few lines to plan out neck/shoulders/spine/arms and legs. Then I go back up to the head and start filling it out with details, now that I know where all the arms and legs are supposed to go, and how big everything is supposed to be in comparison to everything else.
Like so:
Proportion rules are nice to learn and good to keep in mind, but they aren't always true. For example, when drawing small children, the proportions are different than for adult human beings - the head is comparatively larger, the limbs are shorter, etc.
Here's a quick sketch I did for a friend of mine, explaining adult proportions. Unfortunately, the scribbled notes are all in Swedish. but the gist of it is that it helps to think of the hips as a box, and the ribcage as a slightly oval sphere, one on top of the other.
The little dotted lines on the figure on the lower right are proportion lines. In a symmetrical human body, the distance between the eyes to the center of the collar bones/top of the sternum is equal to the length between top of the sternum to the bottom of the sternum ( orthe center of the ribcage), is equal to the length between that central point to the bottom edges of the ribcage, is equal to the length between the bottom of the ribcage to the top of the hip bones, etc., etc.
Again, these proportion-rules aren't always true, but they're a nice rule of thumb to check your figures against if you're struggling with the shape and proportions.