The number one item to bring along when meeting a baby for the first time is clothing. Clothes are something that a baby cannot have too much of. In the first year of their lives the grow rapidly, going through new sizes like crazy. When you buy clothes, make sure you know how big the baby is. Baby clothes go by weight and length, so you do not want to get a 9 pound baby newborn sized clothes because they will be too small since newborn only goes up to 8 1/2 pounds. My son was an 8 pound baby and after I had him, someone bought me a newborn sized onesie; they obviously did not know much about sizing information, so I think it is important to get that message out there. You can also get them clothes for the next season in a size or two larger than the size they are currently wearing. You can figure out the size you need by estimating when the child will be able to fit in them clothes and add 2 pounds per month, since the average newborn grows at a rate of 2 pounds per month For example, if it is January and the newborn is 7 1/2 pounds and in newborn sized clothes, you should by size 3-6 months for spring and summer time.
Bibs and burp cloths are also a hot item. Babies have several feedings a day and tend to get messy. They drool, spit-up and even projectile pee and poop, so it is safe to say that your gift will get used if you go this way. Many people tend to buy bibs and burp cloths in conjunction with clothes, as they are generally cheap. There is no real rule of thumb to bibs and burp cloths. Just use common sense and do not buy girl themed ones for a boy and vice versa.
Other items that are helpful are bottles and pacifiers. However, if you buy these items at least make sure they are using them first. Not all parents use bottles and pacifiers. Some moms exclusively breast feed for the first year of life while others breast feed and use bottles. If the parents are using bottles, be sure to find out the kind they are using as once a baby is used to a specific type of bottle they will not take anything different. The same rule of thumb applies to pacifiers. Some parents do not like to use them, so be sure to check before you purchase them.
No matter what you buy, shopping for a new baby is a fun experience. Just be sure when you get invited over to meet the new baby, you have something special for them.