First and foremost, do not read any New 52 comics, which I will point out, before the pre-reboot comics.
Now, Joker’s origins — He has no solid backstory. Origins, schm-origins, but here are some of the several interpretations you can choose from. It’s not necessary to read them all, but it helps you understand what a mystery the Joker is, even to himself.
- The Killing Joke
- The Man Who Laughs
- Batman #168
- Lovers and Madmen
- Batman #0, 21-24 aka Zero Year (New 52)
After that, you should read:
- Batman #1
- Batman #251 (this issue defines the Joker we all know today)
- Batman #426-429
- Joker: The Devil’s Advocate
- Under the Red Hood
- Hush
- Mad Love
- No Man’s Land; Azrael #53, Batman: Harley Quinn, Batman #570, Detective Comics #737, Shadow of the Bat #93, Azrael #60, Batman #573, Detective Comics #740, Batman #574, Legends of the Dark Knight #126, Detective Comics #741
- Gotham City Sirens #19-24
- The Last Laugh #1-6
- Salvation Run #1-7 + Catwoman #75
- Batman #879-880 (I would recommend reading this whole arc, actually)
- Legends of the Dark Knight #142-145
- Joker, the graphic novel
- Birds of Prey #17, #120-124
- Gotham Central #12-15
- Batman: 80 Page Giant 2010
- JLA #11-15