You have to practice a lot.
How good a drawer are you? If you can't draw yet, I suggest looking for the book "Drawing on the Right Side of the Brain" by Betty Edwards. It explains some base things that are so simple most drawers forget they didn't always know. It's very basic, and the book promises more than most people get out of it, but it's still very good.
You also have to practice a lot.
If you aren't familiar with perspective, I suggest eventually reading about that as well.
In case you want to draw a lot, PoseManiacs is a great site that helps you do that.
http://www.posemaniacs.com/pose/thirtysecond.html
Drawing fast. If you want to be able to draw from real life, you have to learn to capture the intent and the basic shape and the basic movement of a person very, very fast. Making lots of quick drawings also helps you just draw without thinking about it too much, which (according to Betty Edwards' book) will give you better results.
http://www.posemaniacs.com/pose/negativespace.html
Negative space means the space between shapes. When you draw an outline of a chair, you also drew the outlines of the shapes that are not chair. If you draw an outline of a window frame, you also drew the outline of the window's glass pane. Seeing the negative spaces will help you draw profiles and silhouettes.
http://www.posemaniacs.com/pose/randomviewer.html
Random pose viewer without time limit. You can enable a grid, which helps you compare relative lengths and such.
http://www.posemaniacs.com/tools/perspective/pv1.html
One-point perspective visualization tool. I don't read Japanese, so I have no idea why they have this on their site.
http://www.posemaniacs.com/tools/handviewer/
Hand viewer, where you can practice drawing hands. The first object is a man's torso, for some reason, but just choose something else from the menu in the right. You can rotate the hands be left-clicking.
I started practising drawing about, hmm, perhaps two-and-a-half years ago. Back then, I couldn't draw at all. I'm very proud to tell that I'm already
a poor drawer, a great improvement.

If I had drawn more, even 2 hours/week, I'd be more than a poor drawer. So start drawing. If you get bored, try drawing without an eraser, when try drawing with a pen, then try drawing with your left hand, etc. Just keep drawing.
You can learn by copying, but if you only copy you'll only learn to copy. So draw more than you copy and/or trace, if you do that at all.
Finally, this guy is my favourite artist ever. Others might be better, but he's the reason I wanted to learn to draw. He can draw stories and jokes into a single image, and that must be fun.
http://www.elfwood.com/~andersson/profile.html
Here's one of his images I always link to in threads like this:
http://andersson.elfwood.com/Some_sort_ ... 24382.html
P.S. You might want to draw a lot.