What is the best way to learn how to draw?

First -- Two of the biggest mistakes that people make when they start drawing is a) not drawing from life/reference and b) not actually drawing what they see when they are drawing from life.  You see, being people that experience a hundred million objects every day, our brains automatically begin to categorize what we see into shapes that may or may not actually be the way we think they are in our head.  When you draw a cone, for example, you are trying to draw that cone as a shape into whatever sketchbook or paper you're working on.  In order to draw correctly, you have to override that categorizing of shapes, and literally draw the lines and shades that you see in relation to other lines and shades.  What shadows do you actually see, versus what you think you see?  Many people hold up a pencil horizontally in front of them so they know where the edge of the paper will be, in relation to where it appears to be in say, a still life.  You can also measure the distance from one part of the object to the other, based on the relationship between the two, using said pencil.

Second -- If anyone ever tells you that drawing is easy, they are lying to you.  No unpracticed person can draw perfectly, period.  Why? proportions are something practiced and learned, not assumed.  The common joke among students in art school is "the impossible limb."  Because there will always be someone who makes a limb that is either too long, too short, or bent in a direction that is physically impossible.  The same thing goes for objects, but on the less extreme.

Third - Vanishing points... these are the most important thing in the world, so make friends with them sooner rather than later.  The normal way to draw objects in space is by using either one or two vanishing points, depending on what you are drawing.  These are known as one-point and two-point perspective.  As illustrated in my horrible drawings that I didn't use a ruler with...

I'm sure there are other things that I've forgotten to say, but this will get you off to a good start with the foundations.