When you search availability for a flight be sure to indicate the number of travellers in each age category.
For domestic travel, children under the age of 2 years without a seat fly free of charge. Child fares may vary for children under the age of two years with a reserved seat, and may be the same price as the adult fares. On international flights, infants without a seat will commonly be charged 10% of the adult fares. International fares vary for children under 2 years old with a reserved seat.
If you have purchased a reserved seat for your child, it's best to bring your carrier. If you have booked your infant as a "lap child", bring your child seat/carrier to the gate. Airline personnel will store the carrier with luggage if the flight is full. You will then pick it up at the gate upon arrival.
Some airlines provide discounted prices for children but in most cases the ticket price is the same as for adults.
You may be asked by the airline to prove the age of the infant, so a copy of the child's birth certificate is essential.