By 3 months: Your baby should begin to develop a social smile, smile at the sound of your voice, watch faces intently, coo, and make other noises.
By 6 months: Your baby should recognize you and seem happy to see you, seem interested in different sights and sounds, and begin to babble.
By 9 months: He should engage in and enjoy back-and-forth interactions, smiles, and other facial expressions -- and respond to his name.
By 12 months: Your baby should point to and reach for things, wave, and say one word in addition to "mama" and "dada".
By 18 months: He should say 10 to 25 single words, point to objects that interest him, and bring things to show you.
By 24 months: Your baby should say at least 50 words and use two-word phrases ("Doll mine," "Daddy go").