I can't find anywhere how you generate the stats for the characters in Apocalypse World. Is it even part of the rules or can you just write whatever you want. If so, why would I put -2 in something when I can have +3 in everything.
Follow the instructions in the playbooks. Step two (after NAME) is STATS, which tells you "Choose one set:" followed by a list of stat arrays that are unique to each playbook.
The playbooks were designed so that all the rules for players are in the playbook and the Moves sheet, and the rulebook is really only necessary for the MC. Playbooks are essentially guided pick-lists: do exactly what the playbook says, to the letter, and you'll have a character ready to play in a matter of minutes.