A baby should start speaking, or making sounds let's say, at around four months. A baby will start cooing. At around six months the baby will start saying the mm, mm B's and M sounds, and some vowel sounds. Most importantly, if there is speech delay, you want to have a baby's hearing checked. And parents always swear that their babies can hear. But it is really the most important component of speech delay.
If you believe your baby is speech delayed, or your pediatrician has a concern that your child is speech delayed, the first thing to do is have a hearing screen done. In most settings, it will be done in a ENT, ears, nose, and throat office, where the child's ears will get checked, and audiology tests will be performed.
All babies have a hearing test done at birth. That does not mean that the baby will always hear well. So just because a baby passed a hearing test at birth, does not mean that your baby has perfect hearing. So if there is a concern, you should address it with your pediatrician. In addition, parents are usually on the money when they think their babies cannot hear. Most parents who realize that their babies don't turn to sound, or don't startle to sound, are right about hearing loss. And you should definitely bring that to your pediatrician's attention.
A baby should be saying about one word at age one, 12 months, and putting two words together at age two. So, at age two it might be "Give me." or "It's mine." The good rule of thumb is that at one year, parents can understand about one quarter of what a baby is saying. So one over four. At two years, a parent should be understanding about half of what a baby is saying. So two over four. 50 Percent of the baby's speech is intelligible. At three years, three quarters is intelligible. And at four years, a child's speech should be fully intelligible to all people.