Not to be pedantic, copyright infringement is a determination made by a court. A question of fact for a jury or judges -- depending upon the jurisdiction. This is an important detail.
Just drawing characters that are similar to existing characters that other people own is not copyright infringement. Your work viewable by the public -- whether you are making money of it or not.
If you distribute/publish your manga -- whether you give it away for free or try to sell it, using characters that are remarkably similar to existing characters in existing works you may very well might find yourself facing a legal action.
Again, just drawing your manga isn't in and of itself a copyright infringement. It is a good idea to learn how to write or draw or create stories using existing elements. This lets you focus on learning specific aspects of drawing, writing and storytelling rather than trying to digest the entire subject all at once, which can feel overwhelming.
If you did publish or disseminate your work to the public as a parody of the original characters, then you might have a solid defense if the owners of the copyright and/or trademarks brought legal action against you. Mad Magazine was never successfully sued for infringement and their entire business model was based on reusing other peoples stories.
To get a sense of how loosely infringement can be interpreted, the creators of Shazam (Captain Marvel)/Billy Batson were successfully sued by the copyright holders of Superman, even though there is very little physical resemblance between the two characters -- only muscular with short black hair with a little curl in front. The characters have completely different back stories. The only similarities I can figure out is that both are orphans and they have similar powers. A jury was convinced that Shazam ripped of Superman and that is all that is required.