As yousa said.
You need to pick up a book about anatomy. But you dont really have to get a book. You can use google to find everything you need.
I also notice that you don't seem to know what you're doing when you draw that circle when making a head. It's like you do it because someone told you do and have no idea why.
I have no intention of explaining every aspect of guidelines and such and that's why we're telling you to go get some books or whatever on anatomy, They are everywhere some free some not.
Do not be frightened by reading whats in these books, I know some others as well as my self get really bored and turned down when i see several pages covered in nothing but text. But this text is gonna teach you more than any of the exercises in the following pages of any good book.
When you draw;
Think about mass and how it would look in 3D. How would an arm look if a person was scratching the back of his head? (look in a mirror)
What is underneath? You don't draw a skeleton to learn how to draw a skeleton you draw it so you can understand what is under all that flesh skin and clothes. When you draw any part of the body think of how the skeleton looks and how the muscles move around that area. (There is so much.. simply put get a nice 50or so page book on anatomy)
"Sadly, most artists prefer to give the elite their attention."