Major modeling & fashion markets agencies

Paris, Milan, New York, London are the cities for models. They are the capitals of the fashion and advertising industries, which leads to a high demand for all types of models. The modeling agencies there compete to fill that demand with the finest talent . When an agency sees someone who has potential or someone who satisfies the demand, they will invest in that individual to get him/her ready for the market. They don't do this to be nice people - they do it because they feel they can make money from that person. If they found you to be a promising model they can invest some money as a loan against future earnings to prepare you for that market. If they guess wrong and a model do not become marketable and profitable, they will cut their losses and drop you.
The top agencies are working with big-budget ad agencies and fashion designers so there is money available to develop new talent. These top agencies will help train you (more like on the job training), get you test shoots, layout your portfolio, and put together comp cards and other printed materials you need. They take care of finding you work, booking the jobs, bill for the jobs and eventually cut a check for the work you do. Once you are in an agency like this all you have to worry about is following instructions and modeling.
Read info about different modeling markets.

Secondary market agencies
Within a secondary market, the modeling agencies are not working with big budget clients like in Paris or New York City. In some cities there may be enough work that an agency can be a full time operation - perhaps even supporting a few people on staff but there is not enough work to pay for the development of new talent. For this type of agency you must provide the marketing tools and training. The agency may help you with where to go to get photos, composites and training, but they can't afford to loan you money. There just isn't enough budget for them to guess wrong even once. So it is up to you to foot the bill. This is fine if the agency is really out there looking for work for its models and there is enough of a market to make a living. The problem I have found with some of these agencies is that they have no idea what they are doing. They are having a great time playing at being a modeling agency, but you could do better marketing yourself.
What if you do find a good agency, what will they do for you? A good agency will help manage your career and find you work. In big cities this can be split into two parts, managers and bookers. The manager will help you develop your "look", your modeling skills, and develop your natural talents. They will help in preparing your marketing materials for the market in which they think you will be most successful. They should have knowledge of the market place and will honestly evaluate your potential for that market. They should have contacts so they can send you to photographers, graphic designers and printers to prepare your marketing materials. They should have the means to prepare you to be a model. The booking side is where the agency tracks down who uses models. The agency has a phone that is manned and an office that a client can visit. They make the rounds to keep in touch with those who use models and provide them with composites and head sheets of their talent. They advertise, do lunches, do cold calls and turn over stones to find who is hiring models. They book time for models to work, give direction to get there and make sure models show up. They handle billing, collections, and complaints. A good agency finds you work and earns every cent of their commission (usually 15% to 33%). But, in smaller markets, they are a rarity.
Further on down the ladder you find agencies in markets that are too small to support a full time modeling agency. In these markets the modeling school is usually the full time moneymaker and the agency is just a hook. If you finish the school you get to be in the agency. This can be fine if you like to play at modeling and have the money to do so. In most cases if your city has a population of less then a million it will not be able to support a full time agency. Even in a city of a million you may be better off marketing yourself than working with a so-so agency. When you get to a city with a population of a couple million or more there may be just too much ground to cover and an agency can be very helpful.