rexybaby, I'm also curious. How did you bring your baby home without a car seat?
I'm a certified Child Passenger Safety Technician. I do car seat inspections and teach parents how to properly restrain their kids and install car seats. The first step is usually an infant carrier car seat. Yes, some people start with a rear facing convertible, but depending on the size of the baby, that may not be an option. When rear facing the harness straps need to be at or below your baby's shoulders. Newborns often don't safely fit into convertibles because the harness straps are above their shoulders. This is dangerous in the event of a crash.
When shopping for a car seat be sure to get one with a 5 pt. harness and a front harness adjuster. I've worked with many different makes and models of car seats. My two current favorite infant carriers are the Graco SafeSeat and the Graco Snugride. The SafeSeat is currently the only infant carrier on the market that goes up to 30 lbs. and 32". Very nice seat and user friendly. There are different versions of Snugrides. Some go up to 20 lbs, while others go up to 22, and they don't all have front harness adjusters. (This is an important safety and convenience feature.)
If you're in the US, to find a CPS Tech in your area:
http://www.seatcheck.org
Recommended car seats:
http://www.carseatsite.com
Compatibility database and car seat info:
http://www.carseatdata.org
More car seat resources right here at ParentingForums.org:
http://www.parentingforums.org/local_links.php?catid=32
For those of you who think infant carriers are eventually too heavy to carry.... You don't <I>
have</I> to use it in the carrier mode. As long as the baby is still within the height and weight limits, you can still use it and leave it in the car.