the art skill, some human anatomy knowledge (depending on what type of cartoon you want to draw). theres alot of kinds. look at the comics in the news paper and make note of how many different degrees a realism there is.
be awesome and never use a gag writer. make your own jokes. illustrate your own comics, even if you suck at drawing. be yourself.
you have to be able to draw and somtimes be funny
none just have to know draw a tree and a house!
Pants, shirt, I dunno. Does it really require specific clothes? XD
Classes on animation are offered at most art schools.