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Making characters that look good starts with a basic knowledge of human anatomy. You don't want to make your character look like they have too many muscles or too few, too many joints or too few, a badly disproportionate body, etc. Get a good anatomy book and learn about where our bones and muscles are, how they bend, and where they line up. Drawing a manga character requires a basic knowledge of the human body. The more you can draw humans,the easier would it be to draw a manga. So start with drawing (for practice) your friends and even yourself sitting in front of a mirror. To draw poses for your character, you can take images of yourself doing those poses and  then try to draw your character in those poses with the help of the images. You can also use helpful websites like posemaniacs.com for reference. Try to keep anatomy in mind when you do these poses. You don't want your character to end up looking like a Rob Liefeld drawing. The more you practice, the better you'll get.
Study human anatomy. Draw from life. Practice different, dynamic poses. Keep practicing!