How to learn art is a pretty big question. If you want to learn by "teaching yourself" then I suggest getting books on the basics.
And when you feel a bit more comfortable with your drawing skills maybe you could check out the gnomon workshop.
They have very good dvds, pretty expensive but very good. Tho you should probably not jump right into them if you lack the basics.
But most importantly: practice, practice and more practice like with most skills in life.
Hope this helps in some way.
Oh, and a school is good because it forces you to practice, you can't slack because then you wont pass the tests.