Corinne writes:
We are leaving australia on Dec 6th to travel to usa. ...
...From Las Vegas we plan to fly to New York and MAYBE drive to
Niagara Falls.

In Niagara Falls you will find some attractions closed for the winter,
but of course you can view the falls themselves, although they are
turned down to 1/4 power (in summer they run at 1/2 power -- the rest
goes into hydroelectricity). If you have no problem with crossing the
border into Canada, you will find the best overall view of the falls
from the Canadian side.

There may be or may not be ice and snow all around the falls -- many
people find them more attractive that way -- depending on the weather
that December.

I am really worried about how cold it is over there in winter

You will be traveling in at least three climatic regions: the California
coast, the high desert of the Southwest, and the Northeast. The coast
is well moderated by the sea, but in the other two areas temperatures
well below freezing are a real possibility, and you may encounter snow,
particularly in the Northeast. Or you may not -- it's not possible to
say. If there are freezing temperatures or snow and you've never driven
under such conditions, you might not want to do that drive to Niagara Falls.

You should also be aware that since it is winter the hours of sunlight
each day will be short, and in the Northeast at least, they will be
shorter than you get at any time in Australia. This web site

http://aa.usno.navy.mil/AA/data/docs/RS_OneDay.html

will give you sunrise and sunset times for any date and for any city
in the US (also for anywhere else, if you know the longitude, latitude,
and time zone).

and also if it would be too crowded in New York before Christmas.

Certainly! But that's part of the fun of visiting New York.
--
Mark Brader | "In my youth", said the sage, as he shook his grey locks,
Toronto | "I kept all my verbs very supple
| By the use of these smileys -- one shilling a box --
| Allow me to sell you a couple?" --John Dean (after Carroll)

My text in this article is in the public domain.