Velvetee is right - it's being photogenic rather than pretty that makes a model successful.
I remember seeing a documentary about teen model Niki Taylor who was a model from her teen years onwards. She was interviewed sitting on a beach, and looked like any other pretty seventeen-year-old, until she got up and walked away from the camera. You could find a thousand girls as pretty as her, but nine-hundred-and-ninety-nine of them could not move with that fluid grace - that is why she is a model, and thousands of other pretty girls are not.
Look at Kate Moss - snaggly teeth, wonky nose, but the camera sees her as one of the most hypnotically beautiful faces in the world.
It's an indefineable 'something' that makes a model, and a good agency will spot it if youhave it.
Avoid at all costs any agent which asks you to pay for a portfolio of shots - they are usually dubious.
Try a few, and get some advice, and don't sign anything!
Good luck.