Models must choose the right model agency to be successful in their profession. If your agent is very popular and active in the field, your chance of success is much greater than when you are with one of the less popular. But, before finding the best casting model agency, you have to know what is the modelling agency. It is an entity that recruits models for their fashion and non-fashion clients.
It is different from a modelling school where you are trained in the basics of modelling like model etiquette, your carriage, your dressing, the type of modelling you do, your health and nutrition aspects, etc. Another category is talent agencies that represent a myriad of talent such as models, actress, singer, photographer, stylist, etc.
While choosing the right model agency for you, you need to be convinced that the agency is really professional and is actively involved in the industry. Also, you should look at the quantum and quality of work they provide to the existing models. That is, by just registering with an agency you don't become a successful model.
You should get the best professional support in an adequate and continuous measure and the agency should be sophisticated enough to groom you. Another factor to consider is the initial payment factor. If an agent asks you to pay any fee to register, consultation and the like, you should be extra careful because no bonafide institution will depend on any advance payment.