I just got my Casio digital piano...its really nice. I love it.

however I have some questions. In order to have good posture, what height should I set the stand at? It seems too high even on its lowest setting, but I also dont have an adjustable bench, i am just using a chair...and sitting on different amounts of yearbooks trying to get comfortable, haha.
Also, where should my elbows be in relation to the instrument? Level with the keys? above them? below?
I am a beginner so I just dont want to get into any bad habits...

Hello. You need to sit with your elbows at right angles to the keyboard, with the stool/seat an inch or two below the keyboard itself. When you play, your hands need to be arched with fingers bent over the keys, rather than flat on the keyboard. If you have pedals, place the seat/stool so that you cna touch the pedals comfortably without getting cramp in the legs (much as you would when ajusting a car seat). Hope this helps. Good Luck!

If you can't manage to get a teacher at the moment then most tutor books start with advice about posture and hand position.

Yes, most tutor books will really help you out. Hope you enjoy your playing.

Congrats about the piano

I have no extra advice, the above it good but i also have a digital piano and was wondering

what do people think the best volume is as to get the most realistic dynamics?

Well the most realistic is probably somewhere near maximum (modern pianos are pretty loud)...but of course half the reason I bought a digital piano (along with price and space!) was that I could practise at midnight and not get an eviction notice!

One thing I would add about digital pianos: it's really worth budgeting to get a solid, 4-legged stand (the only one I could find was the Quiklok WS-550 at £100). This is because when you're using your digital piano as a practice instrument doing things like finger-strengthening exercises, even a double-braced X-stand will start to wobble about like crazy.

I'd also really like a proper adjustable piano stool, but these seem very over-priced (£200+).