If you want line number to start at 0, put this in your emacs init file:
(setq linum-format (lambda (line) (propertize (number-to-string (1- line)) 'face 'linum)))
For detail about linum-mode feature, see: New Features in Emacs 23.
Setting line number to start at 0 is useful because some editors do that. See: bug#4274