This question is RIGHT up my ally! Go to your local book store, there should be books, also you can watch YouTube videos like jimmyv said, and then practice on pictures that are sort of the style you like, or from your favorite anime/manga. You must remember though, you won't be good at first, in fact it might be REALLY crappy like mine, just remember practice is key,and don't give up!
Practice your ass off and learn to look at your work with a critical eye. When you look @ your work do you honestly think it adds something to the genre? If not, practice more. If so, put it where people that don't know you can see it. Take any critcism with a grain of salt.