Most pediatricians suggest that you wait until your baby is 4-6 months before you start feeding them baby food. If your baby can sit up with support, hold his/her head up, and is interested in what you are eating then you might want to start feeding your baby. Babies first food should be one of the cereals mixed with a bit of formula. To learn more about feeding your baby, please see Mayo's website at http://www.mayoclinic.com/health/healthy-baby/PR00029. It also best if you consult with your babies doctor also.
From the age of 4 to 6 months a baby can hold it's head up and sit well in a high chair. The baby should also be able to make chewing motions and has interest in food. If the baby seems hungry after 40 ounces of formula in a day and is teething, the baby may be ready for a teaspoon of pureed food or cereal mixed with breast milk or formula. Your baby's doctor can advise you on a full schedule for feeding your child.