Don’t wait for anyone’s permission to start chasing your dream, and don’t wait until you think you’re good enough. Just fucking do it. (that should have been Nike’s slogan) And don’t waste another nanosecond of your life drawing sample pages for Marvel or DC. That shit is a waste of time, and it sends a signal to the editors that the only way you’re going to draw comics is if someone hires you to do it. It shows you’re waiting for permission, if that makes sense. If you really had the drive to be making comics, you wouldn’t be drawing 3 pages of Batman, you’d be drawing 100 pages of your own thing. A decently-drawn completed book is ten times as impressive as the most beautiful 3-page sample in the world. Work on your own book (or web strip or whatever) with your own characters. You’ll get more out of it. And even if it doesn’t end up working out (and, to be realistic, it probably won’t), you’ll have a stack of comics to show for your efforts rather than a stack of unpublishable pages of Spider-Man fighting Wolverine.