That you need to feed the baby around the clock, and that the feeds between 11pm to 2am are really important to get your supply going in the early weeks, so forget about sleep and if you are determined, ignore the well meaning family members who insist on giving a bottle to let you sleep. Grit your teeth and go sleepless.
That family can be very concerned about co-sleeping, which is often a necessary arrangement with the cluster feeding and two hourly feeds that go with breastfeeding. If you have the baby in a crib in another room you will end up going insane. This goes completely against the training of American mothers of previous generations, so if you have an older MIL you may expect much flak about the marital bed.
That after a few months you can get a good Medela pump or other good brand that can take the pressure off.
That well meaning family members often prefer to hold the baby while you get on with housework. Learn to tell them what you really need without going ballistic (which they will interpret as all sorts of problems associated with breastfeeding.)
That you will not get much help with breastfeeding in an American hospital even though they may have a 'breastfeeding consultant' complete with clipboard and nametag with her surname. Join La Leche League and try to find a breastfeeding educator in your area (maybe through LLL). For problems like tongue tie, you may well be on your own as far as the hospital is concerned. Find a pro-breastfeeding pediatrician. Ask through LLL (whom you should establish contact with while still pregnant) when you are researching pediatricians. When researching pediatricians, ask about the quality of the nursing staff in the office too -- you want an attitude that they will gatekeep well and not have you come to the office for every little thing (and then charge you through the nose for issues that could have been dealt with by knowledgeable nurses over the phone for free).
That a good latch can be achieved by forming your spare hand into the shape of a letter C and squeezing your boob from underneath and above, making the nipple easy to insert as far in as it has to go (which is quite far in). Your hand should be close to your chest, not out near the nipple. Was taught this by a nurse originally from Africa. Your nipples may still feel as if they are being sandpapered, and one good way to cure them is to let them air dry -- close your blinds or curtains 
.
That if your baby loses more than 10% of birthweight in the two weeks after birth you will have a lot of pressure to give formula. Do what you are advised to do. Jaundice is a potentially serious issue. You can breastfeed at the same time and can eliminate the bottle when the crisis has passed.
That you shouldn't bother buying special breastfeeding tops. A loose T-shirt that can be pulled up and then sort of draped over the baby will be fine.
That public breastfeeding is legal and no-one and no manager of a business premises has the right to make you go to a separate room to breastfeed.
That it can be difficult to ask for help if you need it when family is negative about breastfeeding. You may feel the baby blues are tipping over into PND but feel they would automatically blame the breastfeeding. Develop support systems outside of the family if they are really negative so that you will have someone to lean on if the need arises.
That though fathers can feel left out, when it's the breastfeeding mother and the baby against the world it is far easier if the father stands squarely with you on the subject. Make sure your DP knows how much his emotional support will be needed, and how much practical support he can also give (and don't tell him he's hoovering wrong or not folding the laundry right, etc). There is so much besides feeding that a father can do -- burping, rocking, changing nappies. Around the house, making a decision in the morning about what's for dinner that night and not waiting to be told by the exhausted chief executive is often just as helpful as doing the actual cooking, though doing the cooking is massively helpful too, plus of course all the washing and clearing up.