Varies by child. Although there is much variation from child to child, the following are some accepted milestones. Cooing by 2 months, consonant sounds and babbling by 9 months, 1 recognizable "word" by 13 months, 4-6 words by 15 months and 10-12 words by 18 months. At 2 years a child should make 2 word sentences and be 50% understandable to strangers, 75% by 3 years and close to 100% by 4 years.
Babbling to words. The normal developmental pathway is from cooing to babling to words to sentences to speech.