Unless you want to be confined to your house while your child grows up, you will likely need a car seat. Selecting the right kind of car seat for your child can be very important. Children of different ages and weights have different recommended car seats that can be the difference between life and death in a car accident.
Generally, you should start your baby out in a rear facing car seat. That simply means that the car seat faces the rear of the car. Many such car seats act as baby carriers that can be detached from the base of the car seat.
Later, when your child is older and taller, he can switch to a front facing car seat. Whatever car seat you are using, make sure that the child can fit securely in the straps. If the straps become too tight, see if you can adjust them. If not, you might need a bigger car seat.
Whatever you do, never let your child ride in a car without a car seat. Even if you have a car seat, never put them in the front seat. The backseat in a secure car seat is always the safest place for a child.
For more information, you can reference these guidelines, which will show you what kind of car seat is right for your child: http://www.perfectlysafe.com/carseatregs.html