The reason a child's first word is usually No, is because they hear it so much. I am not sure they know right from wrong at 18 months, but they do know approval and disapproval. A child that small only knows their wants. They have no conception of others, just of their wants and needs. They have to be told no to understand that you are not a decoration set there to please her, and that some actions are not tolerated.
Discipline starts as soon as they stop crying, eating, sleeping and pooping (i.e. when they start recognizing things around them). I am not talking spankings and such. But the word NO is not to be spared at that age.
We started when they were able to crawl. I know that sounds barbaric, but it's not. When they were able to crawl and grab things that they weren't supposed to have, we'd say "NO" and move them/the object away. Disciplining doesn't always mean punishing, it sometimes means teaching right from wrong; letting kids know what's acceptable and what's not. We'd occasionally slap the backs of their hands if they were getting into something dangerous (electrical outlets, hot ovens, things like that) but I can count on the fingers of both hands the amount of times all three of ours got a proper spanking. That's not bad, for three of them....
!
I meant as their only activity. Sorry for the misconception.