This is a general guideline but don't panic if your child is a little bit behind milestones. Often, by the time children are in school they'll be caught up by their peers. You should try and encourage early language by talking to your infant/toddler, describe what they are doing as they are doing it in good detail, use proper emotions, talk slowly and in good tone. Read to them and use sign language when possible.
Before your baby hits 12 months old they should be cooing and babbling. They should slowly be beginning to recognize what certain words mean.
Between 12-15 months babies should be babbling and saying different types of speech sounds. They should start imitating people (ex-"What sound does a duck make?" You say quack quack, baby would say quack quack. Babies should be able to say one or two words around this time. Baby should be able to follow one or two step directions.
Between 18-24 months, most toddlers should be saying about 20 words at 18 months and 50 or more words by the time they turn 2 years old. By the age of two, toddlers should be saying two or three words to form small sentences. They should be able to identify common objects, basic body parts, understand words like "hungry", "Potty", "juice", "done"....etc. They should be able to follow two step demands.
Between 2-3 years of age your child should be using many words in small sentences. Child should be able to recognize colours and basic concepts.
Try no to worry too much as most children catch up with their peers by the time they get into first grade but if you think your child is way behind an early intervention might help. Just go to your doctor or a parent resource centre to get more information.
I've worked in early childhood education for awhile now.