how do you draw a merman? whats the difference.?

Well certainly a male or merman would have male genitals but being half fish they might be concealed somewhat by scales

I'm guessing you mean tail down? Usually the place where the tail starts is a little more narrow, since girls have wider 'hips' on average, even mergirls. This is something I noticed that changes with illustrator, but sometimes the fins look sharper and less flowy on mermen, but that changes by artist and is not a set rule on their anatomy. (Though, it does make them look more masculine.)

a merman has a merdick

Artists typically have exaggerated features they believe are more male, like muscular arms, beards, and angular faces