one way to learn quickly from perspective is to use toys. create a home-made set using household objects as stand-ins (like, say, a clock as a chair) and seating the doll on it and just looking at the doll from differing angles, doing some quick studies (taking photos can also help)
another is to watch dozens of movies. dozens. study the size relation between people and objects like buildings, cars, roads, etc
when it comes to posing you'll definitely want to people-watch out in public. studying from life is the most helpful way to really learn posing and perspective, but not everyone has the time, resources or money to attend life drawing classes or go out and about to people-watch so you have to improvise sometimes. you might be able to get friends to pose for you too, but be sure to emphasize that someone pose NATURALLY b/c u can really tell when someone's "faking" a pose vs natural
here are some links that might help you out:
posing & gesture
background, grids & perspective
perspective & characters in relation to BGs
a video about foreshortening
im going to emphasize that these links are in no way meant to replace real studying of perspective and bodies! but they definitely help out by simplifying things. i hope this helps! good luck w ur manga!
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