You learn to create comics by creating comics.
That's really all there is to it. It's more important than knowing the materials to draw with, where to submit it to, what size paper to use, or what brand of paper towel your favorite artist wipes up her ink spills with, or all the other questions answered in the cartoonist's materials FAQ. If you draw comics regularly, then you'll learn how to make good comics; if you don't, you won't.
When was the last time you drew a compete comics page or strip? Not a pin-up, not a master plan for your ultimate 500-page opus, not a copy of one of your favorite artist's drawings: a complete strip or page, created entirely by you. If your answer is "er... well, I haven't exactly done one yet...," then there's your problem.
So, with that said, I do have some advice for getting started.
1. Keep your ambitions low. You remember that 500-page opus? Put it aside; you can come back to it later. For now just draw something really, REALLY short - like a single comic strip, or a single-page comics story, or a mini-comic - and FINISH it.
2. Repeat step one. Repeat it many times, in fact. Remember - keep to short and simple projects at first, which you can actually finish in a reasonable amount of time. As you get more proficient, you can move on to SLIGHTLY more ambitious projects: an eight-page comic book or a single week's worth of strips, for instance.
3. Don't worry if your work sucks. You'll learn by doing, and your initial efforts are bound to be less than stellar. Just let your sucky comics collect dust on a shelf somewhere (and aren't you glad they're so short?), and move on to your next project.
4. Don't hesitate to imitate cartoonists you like. If you ever get to the point of making money off your work, then this sort of thing is immoral - but chances are you're years away from that. In the meanwhile, imitation is an effective way to learn.
5. Learn to draw. If you're in school, take all the drawing classes you can, especially life drawing classes - that experience will be gold later on, trust me. If you're not in school, check with local community colleges and the like - often they'll offer low-cost drawing sessions (no instruction offered, but models provided). This advice applies even to cartoonists who aren't planning to use a realistic drawing style.
But the main thing - again - is just to create comics. There's no trick to it; just sit down and draw. Now.
Well, you can find lots of detailed answers to that question by following the links in How-To, but it may be useful to read a short and simple answer first, so here goes:
1. Write the script. It's okay if you don't know all the dialog, exactly; just have an idea of how many panels you're drawing, and what happens in each one.
2. Draw a really, really, really rough layout of the entire page or strip (not just panel one) - like in stick figures on a small piece of scrap paper. It's okay if this is so scribbly that no one on Earth but you can possibly read it. All you want to do here is figure out what needs to go in each panel, in what order, in order to make your strip or page work. There's a good chance you'll end up having to erase and restart a lot - which is why you want to stick with simple, stick-figure drawings.
3. Once you have a stick-figure layout you like, redraw the entire layout - still being quite sketchy - on the piece of paper you'll actually draw the comic on. Before you draw enough to feel attached to anything you've done, lightly pencil in all the words, and draw balloons around the words. It's common to find that the words take up more space than you planned, meaning that you'll have to make your drawings a bit smaller in order to fit everything in.
4. Once you have everything loosely sketched and in place, finish the drawings. Later on you can worry about being "professional" - drawing with the right materials on the correct-sized paper and all that - but for now, just use whatever you're comfortable with (pen, pencil, marker, whatever) to turn all these loose sketches into finished comics panels.
Once you finish a panel, don't keep "fixing" it - just leave it alone. Your goal here isn't to produce a perfect page (or strip); you just want to complete a single, imperfect page, so you can put it behind you and move on to the next single, imperfect page. You'll learn more faster (and get more done) by drawing many imperfect pages than by constantly redrawing the same damn page over and over.
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That's it!