There are no real agencies in Phoenix; they are all in New York, LA or Miami, with a few branch offices in Chicago, Boston and Dallas.
Local agencies are almost always scams that don't get you paid work. But you don't need to use a local agency.
It is really very simple.
If you are 5' 9"+ and 34-25-34, and even reasonably pretty, you almost certainly can be a model. Girls like that are very rare, and very much in demand. You just need to send 6 pictures to the agencies (they all want the same thing; links below). Three pictures are full body shots, in a bikini; front, side, and rear views. No posing, no distracting background. The other three hard head shots, front, side and 3/4 views. Hair up, no makeup, no posing.
If you are not 5' 9"+ and 34-25-34, even if you are stunningly beautiful, then you almost certainly cannot be a professional model. There is no such thing as a "petite model." Here is what they do:
Petite:
http://www.anntaylor.com/catalog/product.jsp?productId=30029&N=0&Ntk=KeywordSearch&Ntt=leapord+dress&Nty=1&Ntx=mode%2Bmatchallpartial&found=4&defaultSizeType=Petite
Tall:
http://www.anntaylor.com/catalog/product.jsp?productId=30027&N=0&Ntk=KeywordSearch&Ntt=leapord+dress&Nty=1&Ntx=mode%2Bmatchallpartial&found=4&defaultSizeType=Tall
They use the same models for everything; print, runway, commercial, whatever. All models have to be qualified for runway, the best/prettiest/most interesting of them get chosen for print and commercial work.
