Meconium fills up your baby's bowels throughout pregnancy, and your baby starts ingesting small amounts of the amniotic fluid she swims around in while you are gestating her. Meconium is thick, extremely sticky, and is not at all unlike tar. Hopefully, you will notice the fluid in the first few days following birth. Your baby will start peeing very soon after she arrives in the world outside the womb, but poo may take a little while. Sometimes, you'll be in hospital and have the pleasure of a nurse helping you clean meconium. If you are the one to change that infamous first diaper, don't worry. It is indeed a bit tricky, and your baby may well require a full bath to get all that sticky substance off her butt. Remember not all diapers will be like this!
The next poos will be much easier to deal with, and will be far less sticky. Infant poo, especially when you are breastfeeding, is think and does not smell very badly. Some babies expel meconium during labor and birth, either early in labor or literally on their way out. This alone does not have to represent a medical problem, but it can be one of the signs of fetal distress. Your healthcare team will be on the look out for others if this happens to your baby. Good luck! Enjoy your new baby and don't worry about diapers!