The center of a modeling career focuses on having representation by a modeling agency. There are other ways for unrepresented models to find work, but the role of a modeling agency should be understood by the model to compliment their career. The easiest comparison of a modeling agency is to the role of a specialized employment agency. Their specialty is finding jobs for models (a.k.a. talent) and finding models for jobs (a.k.a. clients). This may sound basically easy, but it's in your best professional interest (as a model or parent) to learn the role that a modeling agency plays in this industry & how it may or may not work for your situation. Don't forget about looking at the "Big Picture" of how it affects all of the agency's models, employees, and their clients.
There are literally hundreds of reputable modeling agencies in the United States. In most states these agencies are governed by laws of their state and must be licensed as a private employment agency. Even their employees working in their agency may be required to be licensed, too. This is best for your overall personal and business protection. You should find out your state's requirements because they certainly vary from state to state. There are standards that the agency must submit to in these cases such as being bonded, business background and financial checks, and copies of forms and contracts that will be utilized by the agency. This manner of screening also assists in weeding out some of the people involved in scams and poor moral business practices that just keep changing business names to stay ahead of their shady pasts.
Another requirement for an agency needing a license is if it is in the business of working with unions like SAG (Screen Actor's Guild) and AFTRA (American Federation of Radio and Television Artists). These are the industry standards for models and actors, and an agency will be franchised as such usually quite clearly. There are other associations such as Better Business Bureau (BBB), that can be a resource for seeing if there have been a lot of complaints and unsettled disputes with an agency. These sources can give you a general idea to whether or not this agency is reputable in the way it is accountable to its business practices.
Now, after that initial prerequisite to look into and/or understand, here's the next dimension to appreciate. Where is the agency? What is that city's "market"? For example, how many and what kinds of clients are in its market? Are they Fashion clients? Are they Commercial clients? The market is a reflection of the kind of work that an agency may be likely to offer its models. (Current trends for markets throughout the United States and International markets and the role your modeling agency can play in your diverse representation is also what a model needs to consider.)
A modeling agency can range from very small to very large, and even the types of models that an agency specializes in representing can be just as unique, so with that said, they know what they are looking for. People outside of this industry may think, "How hard is it to find a model?" Well, there are many ways that agencies see potential models, but they evaluate a model's potential dependent upon how much money they anticipate that model earning while at their agency. The agency knows who their clients tend to be, and how many similar models they may have, so it's a matter of supply and demand.
All modeling agencies have some form of interviewing and evaluating new models, so you can easily find out first by checking their website or calling and asking what their procedure is for accepting new models (ex. Will they be holding any "Open Calls"? Do they want you to just send a picture? etc.). There's usually a time when the agency opens its doors for anyone to come in to be considered for representation. Most agencies will specify when they hold their open calls, or where you can send some snapshots or composite cards of yourself if you live out of their area. There are many other ways that agents can see new models and talent, too, such as referrals from scouts, other models, photographers, modeling conventions, modeling schools, pageants, modeling agencies from other cities, websites, and even clients. Some scouts are actually employed by a specific agency that knows what qualifications that their agency is looking for versus an independent scout that is able to receive a finder's fee (and may even be eligible for a percentage of the model's future earnings). Not every "discovery" is compensated by money, so it depends on the relationship of their association to the agency.
A modeling agency must be very selective to whom they represent. They may see hundreds of models, but there are industry standards that the agency must meet in order to fill their client's needs. This is where your "look", height, size, gender, experience and "market" are considered. The model is part of a group of individuals that work as a team in getting the job done, but there is usually more at stake financially for a client (and agency) when it comes down to either hiring the right model or going through the process of rejecting them. Business is business. Remember, an agency takes a commission out of the model's rate for getting them the job, and they get a fee from the client, too, because they found them the model...(a.k.a. employment agency). Clients are the ones who select the model, so it's in the agent's best interest to find the right models because it's a win-win situation for everyone.
As much as agents are always looking for new talent, there will be different standards of how much one-on-one training will be offered to the models it represents. It is in an agency's best interest to make sure that their models that they are sending out on different jobs are up to par on the most basic requirements needed as a model. Agents can face a public relations nightmare (a.k.a. professional embarrassment) when one of their models represents their agency poorly. Some agencies may have a general manual that they hand out to all of their models that list their policies and standards that they want their models to follow.