Really the only advice I could give you is to practice your ass off. Set reasonable goals for yourself, and don't expect to be able to draw a live portrait within 2 hours on your third month. It takes many many years. I know that kinda sucks, but there really is no magic trick or shortcut to it.
But hey, if you can accept the fact that most things you draw will suck for the next year or two and still find the dedication to keep going at it, you can achieve great things. Just look at where I'm at now [http://artbyruben.blogspot.com] (the top post is ZBrush digital sculpting which may not interest you, but there's some good old pencil-and-paper anatomical study and a bunch of portraits further below).
I've never had any classes or lessons. It's all just practice, ever since I was able to hold a pencil.