Depending on the age of your child there are several different types of car seats that you will need.
Age is less than 1 year and weight is less than 20lbs you should use an Infant Seat.
Age is at least 1 Year and Greater than 20lbs you should use a Convertable Seat.
The AAP (American Academy of Pediatrics) recommends that you keep your children in a convertable seat until they have grown out of the seat then move them to a booster seat until they are 4'9" or 12 years old.