First things first - take a deep breath and let it out. Now, go to the store and buy yourself snacks and a magazine. Throw a comfortable nightshirt in the cart, and maybe go ahead and throw in a baby book or a blank journal. Grab that chocolate bar you've been craving and get some ice cream too. Do you sense a theme? Spoil YOURSELF.
Once the baby arrives, you will be up to your ears in baby stuff. You will lose sleep, have nearly every minute of the day accounted for, and probably have too many things on your to-do list then you can ever remember there being. But that's okay. Being a mom is one of the best feelings in the world. The first time they look at you, smile, giggle, grab your hand, and call you "mommy."
But until this child is born, you are able to do the things that you've always taken for granted. Take advantage of this being your first pregnancy and milk it - you will look back on this time and smile when you remember how wonderful it felt to sit on the couch with the gallon of ice cream at 3pm and not think about when the baby is getting up, or when the next feeding is, or any other concern a mother has flying through her mind at any given point. Enjoy this time!