"Once a child starts crawling and getting into things, you'll have days when you feel like the only word you say is 'no,' " says Traci B. Pitts, a child psychologist in Reno, Nevada. But while your child may understand the word itself as early as 6 months, she won't be able to stop what she's doing in response until much later. In the meantime, your best bet is to redirect your baby's attention away from the activities you don't want her to do, toward things that are more acceptable.
At around 18 months, you can start to wean your child — and yourself — away from the redirection/distraction habit and simply say "no." Just understand that it may take many months for your child to actually obey your command.
"At this age, choose your battles and try to limit your "no's" to safety issues, or you'll drive yourself crazy," says Pitts. For instance, "You may not play with the knobs on the oven" or "You may not go outside without shoes" are perfectly acceptable reasons to say no, without needing to redirect your child's attention.