Besides what other people have already noted here...
Invest in good tools. ESPECIALLY if you're going to color traditionally. A $5 set of Wal-Mart markers are NOT the tools to use if you want your art to be as good as possible. In that same vein, don't try to improvise if you are just beginning to learn to color traditionally. If you don't have the shade you're looking for, go out and get it -- this is especially true for skin colors, as most marker sets don't include them for some weird reason.
On the other hand, don't break the bank for your art, especially if it's just going to be a hobby. This stuff can get really expensive really fast, so watch what you buy. I'd suggest you start with a good mechanical pencil from, say, OfficeMax, preferably with replacable lead and eraser(yes, I have bought mechanical pencils where the lead couldn't be replaced. You can imagine how annoyed I was.
razz ), a good eraser of any type(make sure it won't smudge your pencil
smile ), and possibly a good Micron or other quality pen. Be sure to research via Google all the tools and their uses. You don't want to be buying a pen that is way too thick for the kind of inking you do.
razz Get realisitic anatomical references. An anatomy book is good, and if you are squeamish about human anatomy there are family appropriate books out there. You can also get posable models -- I have not had them but I hear they are nice.
Please, please, please do NOT start out with anime. It will cost you in the long run, as it did me(I have been drawing on and off for many years and have basically had to start all over because I hit a glass ceiling in my art). Start with realism, and THEN develop your OWN style. I don't buy manga so I can see anime quality artwork that doesn't vary in style from anything else I've seen. Remember, you probably aren't going to be actually animating a show any time soon, so you don't want your art to look like it, since there is a big quality difference between anime and manga.
3nodding As far as copying, I wouldn't recommend the kind of copying most people do -- taking a full, completed picture and eyeballing or even tracing it, and end up with a poor quality, warped version of usually an anime picture(which is poor quality to begin with). Instead, try taking quick, small, anatomically correct, naked(rather, "blank" which is naked with certain R rated parts not drawn in) sketches(I have several in my anatomy book that I use), eyeballing THEM, and building a character on top of them. That way, you get to practice good anatomy AND all the other good things that make up drawing, like clothes, hair, facial features, etc.
3nodding Finally, use references. A LOT of them. I rarely draw anything without my computer, an anatomy book, or a live model. It can get tedious, but as you get more practice you will be able to draw without having to look up something everyone 5 seconds, and you'll have a solid foundation to have very nice art to boot.
smile EDIT: Oh, one other thing. As you start to develop your own style, remember you don't have to conform to ANYTHING, not even a style you made up. Vary things, try drawing things more realistic or less realistic, etc. Getting stuck in a style rut is one of the worst things I think someone can do, as it prevents them from changing, varying or ever getting better. Of course, you CAN have a favorite style you do most of the time, as long as you are enjoying doing art and mix it up from time to time.
wink EDIT2: By the way, I know you said 'manga' and not 'anime' but I wanted to make sure you knew the difference. Most people want to draw anime style because that is what they seen on Cartoon Network, but the fact is that anime is poor quality artwork compared to manga.
domokun Also,
here is one of my most recent pictures. I've been coloring it off and on for many months now.