You shouldn't learn to draw in a manga style. You should just learn to draw, realistically and when you're good enough, you'll be able to draw in most styles and be more versatile than if you start with a manga style which has improper proportions and will put you in habits that will be hard to break.
www.conceptart.org is a great forum. Starting a sketchbook there or posting a specific work you're having trouble with in the critique section will get you sound advice.
They don't specialize in manga, but they can certainly help you become a better artist, and that style will come naturally.
Also, there's this guy's series of videos:
http://www.youtube.com/watch?v=BsQ1WVyPHvU
He focuses heavily on the basics to see the flat paper as a 3D landscape and gives you good exercises to do to get better no matter what artistic path you take.
Unless you already can draw well, and are asking about tutorials for making serial art (comics/actual manga/storyboards/things like that) that flow well. In which case
this is a great book.
As for places to discuss manga and anime, I don't have any idea either.