Draw a block.

Most tutorials suggest you begin with a sphere; but I'm with George Bridgman on this one. A block immediately sets pose, camera angle, and perspective. And, moreover, it's easier to draw than a sphere, particularly a big one.
Note that this block is not a cube. It is square when viewed from the side -- that is, the human head is, conviently, as long front to back as it is high top to bottom. However, the width is less than the height. Proportions vary greatly from face to face, but as a rule, the head is about 2/3 as wide as high. Manga faces tend to be quite wide, or rather quite short, so you might want to go as wide as perhaps 3/4 of height.
You need to be able to draw this block, in proportion, from any angle. Practice until you can. Then move on to step two.