There's no question whether you're getting one; the question is just what kind and how many. We might as well go ahead and break it to you, you'll probably end up having to buy three separate car seats to cover three different sizes of child: an infant seat, a toddler seat, and a booster seat. The only exceptions are if you buy a convertible car seat that covers two stages, allowing you to buy just two seats, or if you have a newer-model car that comes with its own booster seat, thus cutting out one of the purchases.
This is one of the few categories that the experts unanimously recommend not taking a hand-me-down to save some expense. Car seat technology is always changing, and since it's a safety issue, you'll want the most up-to-date style available.
Rearward-facing Baby Seats Group 0: for babies up to 10 kgs (22 lbs) roughly from birth to 6-9 months, or Group 0+: for babies up to 13kg (29lbs) roughly from birth to 12-15 months
Forward-facing child car seat Group 1: for children weighing 9-18 kgs (20-40 lbs) roughly from 9 months - 4 years.
Booster seats and booster cushions Group 2: for children weighing 15 - 25 kgs (33-55 lbs) roughly 4 to 6 years. Group 3: for children weighing 22-36 kgs (48-79 lbs) roughly from 6-11 years.
Booster seats and booster cushions do not have an integral harness to hold the child in place. The adult seat belt goes around the child and the seat. So it is important that the seat belt is correctly adjusted. The basic points to note are:
the belt should be worn as tight as possible
the lap belt should go over the pelvic region, not the stomach
the diagonal strap should rest over the shoulder, not the neck
When children first move out of the forward-facing child seats into booster seats and cushions, initially, ones with backs may provide a better fit for the seat belt. Booster seats with side wings will also help to prevent injury in a side impact by protecting a child's head, and on several seats, the height of the side wings can be adjusted as the child grows.
Booster seats and booster cushions can be used in the front or rear of the car, but it is safer to put them in the rear, especially if there is a passenger airbag in the front.