1. Breast Pump - Most insurances will cover one now, so you might as well get one if it's free! You may plan on breastfeeding and never pumping, but you never what could happen. For example, baby may not be well enough to go straight to breast so you will want to pump breast milk for baby. If you plan on going back to work, then a pump is a must in order to keep your supply going and to provide breast milk for your baby while you are away. I highly recommend the Medela Pump in Style.
2. Nipple Cream/Balm - Your nipples will be sore in the beginning, and using these creams help. My lactation consultant highly recommends Mother Love and after using both Lansinoh and Boobease, I definitely prefer Mother Love over the other two.
3. Nursing Pads - Once your milk supply comes in, you will start leaking and pads are a must. You can use disposable pads or if you want to be more eco-friendly, reusable pads. My favorite are the Bamboobies reusable pads. They are super soft and way more comfortable than disposable pads. However, Bamboobies are quite pricey, so I also got these off of Amazon. They aren't as soft as Bamboobies or as discreet, but still work quite nicely. They are also thicker, so I prefer to wear these around the house and use the Bamboobies when out and about.
4. Nursing Sleep Bras - These are all I wear. I wouldn't bother getting any other nursing bras. You can just wear nursing pads with these if you are worried about your nipples showing. My favorite is the bravado basics, which appears to no longer be available. The next closest thing would be the Medela Nursing Sleep Bra, which has the same design, but I don't find the material to be as comfortable.
5. Robes - I only owned two robes, but now I own four and find myself wanting more! These are just so convenient when breastfeeding. Robes are great for those first few weeks when you don't plan on leaving the house.
6. Hands-Free Pump Bra - This a must for pumping as it will allow you to hand express at the same time to get more milk out. You can also eat, read a book, or work at your computer. It's just more convenient to have your hands free rather than having to hold the device to your boobs for 15-30 minutes. My favorite is the Medela Easy Expression Bustier. I would recommend having two so while one is in the wash, you have another clean one available.
You may be surprised to see that I didn't include a breast feeding pillow as one of my must haves. I do own a My Brest Friend, which I love using, but have actually found just using regular pillows and blankets a lot more convenient and easier to grab and get ready when in a hurry. In my opinion. a breast feeding pillow, like a Boppy or My Brest Friend, can be nice to have, but are not necessary and you can definitely get by without one.