The FAQ and Known Information sticky's first post gives great locations to EV train your Pokémon based on stat category. As for the best time to start EV training, I'd strongly suggest doing it after you complete the game's main story and beat the Elite Four. EV training while playing in-game could become difficult as you won't have access to all areas, so it only takes you longer.
With that said, if you have anymore questions, please use the HG/SS Help Thread.