I've nursed 4 kids for a year each and the Loving Moments bras from Walmart are my absolute favorite nursing bras. Sure they aren't much to look at, but they are so comfortable, even when your milk first comes in. After your supply is established its nice to splurge on something fancy, but these are great for everyday.
did Jessica Simpson ones are so comfortable I really like them and they're relatively inexpensive
Find a size that fits around your ribs snugly but has a bit of space in the cup. Nothing too tight, not a sports bra, it will hurt once your milk comes in. It needs to be supportive but if the first few days one without underwire or ribbing will be most comfortable. I would say buy one cheap one now and then buy two or three better ones once baby is here. Keep in mind if you breastfeed past 3 months you will likely change sizes again as your milk supply regulates.