Basically, nothing is essential except a carseat! Everything else you've got on you. . .boobs and hands. :) I had really no idea last time either and I didn't buy much, figured I'd get stuff as I went. Found that I really didn't need much at all in the first 8 weeks, after that baby gets more "interactive" and some toys and such are nice, but at first all they need is food and love. Really.
But here is my list of things that are good to have around in the first month or two:
For mom (assuming you will be BFing, I haven't bottle fed so don't know all the stuff you'd need for that):
--breast pads (you will likely leak A LOT at first)
--lansinoh for sore nipples (although I didn't find it essential)
--a breast pump is really nice to have for engorgement or other issues, but they can be easily rented too
--I'm definitely going to have these this time, although some cold and hot packs can work just as well http://www.amazon.com/Earth-Mama-Angel-Baby-Booby/dp/B000JVGR4I
--"Boopy" pillow, although rolled towels or other pillows can work too for BFing
--simple basic clothing, cotton onesies and sleepers, sleep sacks, wool longies, hats
--lots of receiving blankets (you wind up with them all over the house and use them for many
--swaddle blankets! swaddling can really make a difference and having big enough blankets (or the special swaddle one's) and learning how to use them are a must)
--moby wrap or other safe baby carrier (I found that my ds hated the stroller till he was big enough to sit up in it and so I wore him for the first months and found it much easier actually than lugging the carseat and stroller combo!) I wound up with a Moby, a Becco and an Ergo and still want to get a ring sling and a woven wrap for this babe!
--burp cloths! I just use prefolds
--bouncy chair (the vibrating kind). I basically wore or held ds for the first 2 months non-stop, but the vibrating chair was a godsend for those moments I needed to shower or use the bathroom.
--a thermometer is good to have and also a nose aspirator (check out "the nose Frida", sounds gross, but is amazing!)
--obviously diapers, and a it's nice to have a good, all natural diaper rash cream handy (Earth Mama Angel Baby is good). They used to tell you to swap the umbilical stump with alchohol, but with ds they said to just leave it alone.
--it's nice to have a rocking chair or a birth/yoga ball as newborns love that sort of swaying, repetitive motion. some do like those electric swings, but my ds didn't want to be out of arms, so again just a wrap/carrier was the best thing I could have.
that's all I can think of at the moment. . .
of course there is tons of stuff you *could* get and might make your mom happy RE a registry. :) Here are some things that I think are non-essential, but wouldn't mind having/getting last time (now I have most of these already): a baby bathtub, baby bath towel, natural and non-chemical baby shampoo (I rec. California Baby) a diaper pail, changing table/pad, dresser for babies clothes, mobiles, simple colorful toys (I like the Lamaze toys or Haba's wooden rattles), you could start the baby a library even though they obviously won't use it for awhile (so many lovely kids books--get board books!), play mat, "excersaucer" (although they won't need/use this for awhile).