1) Sleep n' Plays are all you pretty much need the first few weeks. The name says it all. Babies can both sleep and "play" in these (play of course consists of a few blinks and jerky limb movements in the early stages). They are warm. They cover their feet (even better, get the kind with little fabric folds on the sleeves that can cover their hands too, works so much better than mits!). They are soft. They transition seamlessly from day to night (for the baby who works then parties and doesn't want to have to change!). Buy a ton of these in the 0-3month sizes (it doesn't really matter if they are baggy, because they don't have a waist) and you'll be good to go for a while. Oh and get the ones with zippers if possible (versus snaps). You will be grateful at 3 in the morning when you are fumbling around in the dark to change a wet baby that you don't have to deal with a million little snaps.
3. Onesies really are the best. Next to the sleep n' plays I will say the onesie is the single most useful piece of clothing the first few months. It's all fine and good to go for cute and stylish when you're going out, but on days at home or to just run errands it's all about the onesie. Easy, comfy, and I feel like babies just like to have as little clothing on as possible (granted this would not work when it's cold out, I was lucky to give birth at the start of spring). And they're just cute, especially when your baby gets plump little legs with delicious folds you just want to bite. I will say I bought waaaaay too many white newborn onesies that Ryley grew out of in like 2 days. The Gerber ones especially are sized down so she pretty much immediately wore the 0-3 month sizes and outgrew those quickly too. And if you're like me and like a little cute factor you don't even need to get the white onesies. Just stock up on the cute printed ones when Carter's has a sale (which is pretty much all the time).
Overall I learned from Ryley not to over-buy before baby arrives (I went a little crazy), especially because you don't know what size they'll wear and for how long. It is possible to buy your baby clothes after they are born (there is such a thing as online shopping). For the first few weeks you really only need some basics and then as time goes on you can start to get a little more creative/fashioned forward. Definitely ask people you know who have older kids to borrow stuff (most people are thrilled to get rid of baby clothes taking up space in their basements/garage/attic if they don't plan to have more kids).
Oh and just one word on those massive headbands with a flower on them that is the size of your baby's head. Don't. I mean if you want to take pictures that you can use to embarrass your child in the future, go for it. But for everyday life, just don't.