How to get The Jokers heath ledgers hair style?

First of all, you'd have to get it to kind of a greasy consistency in order to get it to look like the Joker's hair. I would suggest using a hair pomade, which can give you the wet, greasy look pretty easily. If your hair's naturally straight and you want it to look wavy, you can either braid your hair in 10-12 little braids all over your head (like cornrows) while it's wet, and then when it dries, take them out and the hair will be curly, or you can use a small-barrel curling iron, but that might make it look too "fluffy," and you obviously want that flat, curly look.