I like to use an infant carseat when my children are infants instead of a convertible carseat. I like to be able to fasten my baby into the carseat while inside the house where it is not too hot or cold, like it often is in the car. I also like to be able to take my baby in or out of the car without waking him up to get him out of the car. Often times he will fall asleep just before we get somewhere, so being able to carry the carseat in and allow him to continue sleeping is something I like to be able to do. I also am thankful to have the carseat when out and about if I need to lay him down for some reason such as when he wants to take a nap.
Many people I know like to skip the infant seat and go right to the rear facing convertible carseat. They like to be able to take the baby in and out of the car and carry the baby in a sling or carrier and not have to worry about carrying a carseat around. I also like do this on occasion when the situation calls for it, but I can still do that with the infant seat just by leaving it in the car. It is an added expense to buy an infant seat, but if look, can often be found for $70 or less. Everyone has to make the decision that works best for their family. Regardless of which type of rear facing seat, an important requirement is for the shoulder strap slot should to be just below the child's shoulders. If it is not, the straps needs moved or you need to change to a new seat that fits the child.
I used a Graco's infant seat, but any infant seat will work so I won't bother recommending one. If the baby is bigger, look for one with a higher weight limit. Besides that, just go to the store and test some out to see if any feel better to carry. Some may be lighter weight, which is a plus. Avoid buying a used carseat from a stranger since it could not be proved it is accident free. Once a carseat has been in an accident, it needs to meet specific requirements for it to still be safe. It would not be able to meet these requirements if the seats history is unknown.
The following video talks about what can happen to a child who is placed in a forward facing carseat before their body is able to handle the force of a crash. If also has video showing a crash test with a front facing carseat vs a rear facing carseat to get an idea of what will happen in a crash situation.
Many booster seats only go up to 40 lbs. Others don't have the 5-point harness and require use of the seat belt. The following video shows a crash test with a 5-point harness carseat and a high back booster using the car seat belt. It shows that the seat belt doesn't do the best job of holding the child in place during a crash. Therefore, I choose to use a 5-point harness for our everyday booster seat and I will use it as long as he will fit in it.
An important tip is to be sure the child is not wearing a thick winter coat. When compressed in an accident, a winter coat will leave a gap between the child and the carseat which may leave room for the child to fly out of the seat. My kids either wear a thin jacket or no jacket, even in the winter. They either cover up with a winter coat or with a blanket to keep warm if they are not wearing a jacket. My baby gets to have a blanket to keep warm. It is also recommended not to use any product that did not come with the carseat that would lay between your child and either the back of the seat or the straps. So, if the necks straps were not tested specifically with the carseat, then they should not be used with the carseat. The same goes for any padding that can be found to try to make the carseat more comfortable. If it didn't come with the seat, don't use it.
Another tip is to make sure the chest clip is at armpit level. I often see babies in their seats with the clip so far down that the baby would be able to fly out in an accident. Or sometimes the straps are not completely over the shoulders which could allow the baby to be ejected. The straps also need to be pulled tight enough not to allow slack where baby could slip out. Therefore, the straps should not be able to be pinched. If they can be, the seat should be tightened.
One more point to know about carseats, is that they do expire after the time noted by the manufacturer. Some carseats have longer expiration times than others. The 2 seats I use have longer expiration dates than most other seats. The Radian is 8 years and the Nautilus is 10 years, while most others are only 6 years. I wanted to be able to use my seats for multiple children, so the longer expiration time was a plus on the seats that I chose.
Some people may wonder why a carseat would have an expiration date. The following video shows a crash test using an expired car seat. It shows how the straps broke through the plastic on the back and allowed the child to come forward away from the seat. Included in this video is other information about the proper use of a carseat.
I wanted to say one last thing regarding carseats and their recommendations and laws that are made to keep kids safe. These laws are generally the minimal requirements to try to get people to keep their kids safe. In Illinoisb the law says to keep a child in a booster till they are 8 years old. That does not mean that automatically at 8yo, a child will be able to fit in an adult seat properly. The important thing to remember is keeping the child safe, so if the adult seat belt does not come across the child in the correct place across the chest, then the child still needs some sort of booster seat even if he is 12 years old.
I hope this has been helpful. I will answer questions that are left in the comments.