Get a couple nursing sports bras for now- places like Target and Walmart have them, but definitely wait until your milk comes in to order real bras. Your boobs will thank you for getting a good fit. You would not BELIEVE the size changes that can happen when your milk comes in. I have found that bra sizing is a crap shoot at best- take your own measurements (using a cloth or paper tape, measure around your chest under your arms and above your boobs- that is your band size- then measure around your chest and across your nipples- the # of inches different from the band size is your cup size (ie. 1 inch=a 2inches=b, etc.) This has never given me a perfect fit, but it gets you in the ball park.
I HIGHLY recommend the website figleaves.com- they have the widest range of sizes in nursing bras I have ever come across (and I am a hard to fit 34/36I) and after the initial shipping charge ALL EXCHANGES AND RETURNS ARE FREE!!! I went through probably 20 bras, with 5 different shipments, before I got the one that was just right- all for about $4.00 shipping.
Having been nursing for over three years now, I am glad I got the best fit possible, and sorry I wasted so much time in the beginning wearing bras that weren't quite right.