Your milk doesn't come in until after baby is born and starts suckling. You might
get a little bit of colostrum coming in earlier, it is the consistency of butter or heavy cream when they drink it but I would find one or two little drops dried in my nipples like yellow wax before she was born.
It IS possible to BF with flat nipples but you will have to work hard to get a good latch. It will probably take a couple weeks, but once baby gets latching well it'll get easier. Some people find the nipple shields help and others don't like them at all so just try it out and see how it works for you and baby. It may be worthwhile to hire a certified lactation consultant if you don't have any experience breastfeeders in your family to help you out. Or check to see if there is a La Leche League chapter near you http://www.llli.org/webus.html
thier services are almost always free.