What you do to get accepted into a dance college depends on what YOU want to do. There are college dance programs where no audition is required (typically in a liberal arts college), and others where it's easy enough to get accepted if you're at the intermediate level. It's only at the more selective college dance programs where you need to be at the advanced level. For example, at Barnard College (which is in New York, since you were interested), "Once admitted, a student can take any dance course that is appropriate for their level."
http://www.barnard.edu/dance/prospective…
Needless to say, though, the more serious you intend to be with dance, the more you'll need to prepare. IN ORDER to start researching which colleges you might be interested (as OCU Dance said in passing), you should get hold of the Dance Magazine College Guide:
http://www.dancemagazine.com/thecollegeg…
As you read through the program descriptions and articles, you should also start visiting ANY college dance program, starting now with ones near you. At the very least, go to see their performances. This helps you start getting an idea about what you like about college campuses, the dance facilities, the type of students that go there, and so on. Be sure to read the programs to see what they may say about the teachers and students. You'll need this background in order to make informed decisions about your top choice schools.
To take things to a higher level, you really should be attending selective summer dance intensives. Beyond getting the intense training they provide, those programs also help you to get practice with auditioning, as well as experience working with different teachers and choreographers. They also help you to network with dancers from many other regions, and that network is valuable in the dance world.
For the selective dance programs, it's not so much a portfolio that you need, but a dance resume. The more selective intensives you've attended, the better. You'll also need letters of recommendation from teachers (the bigger their reputations, the better), and you may eventually have to prepare a dance solo. Sometimes a short video of your dance abilities is also required.
Be sure to keep up your grades, as many colleges base their decisions (and award scholarships) on your academic transcript.
My daughter graduated with a BFA in dance in May of 2008 and is now a professional dancer. My field is in college and career counseling.