You can submit your application on the Ford website and attend an open casting call if you wish, but you should brace yourself for the very high likelihood that you will not be signed. You may be an absolutely adorable girl, but I've never known any Ford model who was 5'1. Even for the Teen Division you must be over 5'8. They prefer the women models to be between 5'8-6' and 21 years old or younger.
Wilhelmina will occasionally hire petite models, especially if you are athletic. They have a special division specifically for fitness models, and are less discriminatory about height.
There are numerous casting and modeling agencies in the city that hire smaller girls, so you could try submitting a few polaroids or head shots to them. "Your Friendly Jewish Accountant" is incorrect that you need to already have a portfolio and experience in order to apply to agencies. The majority of girls do *not* have any previous professional experience or formal training, and in fact, the top agencies prefer that their girls have "fresh faces" that haven't already been seen in print ads. My agent once told me that girls who had been child actors, in beauty pageants, or done catalogue work were far harder to train than those who had absolutely no experience because you not only had to teach them how to model, you had to make them shed off the habits they picked up previously that were not conducive for high-end modeling. They want naturals. All you need is a few simple, clear shots of your face and body in order to apply. If you are hired and they feel you need to learn some techniques, they will send you to classes. Never, repeat never, sign with an agency that asks for you to pay a fee or claim that you need to see a certain photographer or take classes; it's almost always a scam. I'm also stunned by Peedlepup's answer since Ford has an extremely active West Coast division, as does Elite and Next, and there are far more casting agencies and boutique modeling agencies in LA than in NY. If you are supermodel material an agency here will most likely send you to NYC or Milan, but you can most certainly apply here.
Good luck!
~ Pax / Peace : )