As a model, you're basically a product selling another product whether it be jeans, make-up, hairspray, or whatever. If you can't move the product, the client will find another model who will.
At the very top of the pyramid are the supermodels. Realize that only 5% of all models get to that point. Most models are quite content with getting steady paying jobs like catalog or magazine work as fit models or parts models.
Models need to photograph well. You might turn heads on the street but if the camera can't capture your beauty, you're finished before you even started.
A model's bone structure is everything. A woman with high cheekbones, piercing eyes, a proportionate nose, pouty lips, and an angular but symmetrical face probably has what it takes beauty-wise to be a model.
Although bone structure counts for a lot, good skin is also very important. Pimples, large pores, and blemishes work heavily against a model.
Your attitude as a new model is very important. First and foremost, you must be confident. Confident doesn't mean acting like a diva. It means confident in your talent but at the same time, acting like a professional and not a spoiled child.
Always be a professional.