It is how good the product is, how it is packaged and how it is marketed that will depend upon the success of a model.
It is a business where some models do get discovered, but most have to work very hard to become successful. It is the hard work and dedication to this business that will determine ones success. One has to invest in themselves and their career.
There is a lot of controversy around paying money to become a model. But you do need to invest in yourself if you want to become one. You need a portfolio and you have two options.
You can choose to have professional photos taken with a professional Photographer, Make-up Artist and possibly a professional Stylist/Art Director or you can have a friend or relative take photos of you for free.
If the product (you) is exceptional and luck is on your side you might get taken in by a modeling agency without a professional portfolio. But when you are competing with many other people to be discovered your chances would definately be increased with a professional portfolio.
Your business is you.