Many girls approach me about getting a modeling agency to work with them, and I get asked about my thoughts on working with certain agencies, but the thing is, YOU have to feel good about it. When you work with people it isn't just about YOU being accepted, it is about you feeling the opportunity really is one for you, feel in your gut it is a good thing for you. Don't rush or feel pushed into anything. Interviewed them while they interview you. There are good agencies out there and professionals who are positive and great to work with, aim for it. Be true to you.
Ask questions, ask about the jobs they book, their clients, will they work with the compcard you give them to work with? what can you improve on? Don't expect a full fledged hands-on experience in commercial print modeling, agencies in print modeling are not as hands-on, but do expect to get professionalism an aim for it. Ask questions. Don't feel you must do anything. If any agency tells you that you must pay to work with them, or that you must purchase a photo package, RUN! you do not have to, but you do have to prepare your own photos, be able to craft and create the marketable photos you need. Managing yourself is a big part of working as a shorter model with agencies.
An art student creates her portfolio before a gallery or company hires her. The same for models,you have to craft and create your photos, make sure they represent you honestly, and the best of you. Create a comp card, buy a portfolio, be prepared, be ready, invest, and work!
When meeting the agency or when an agency wants to meet with you or work with you, prepare questions, ask, and don't be afraid to say no. You will find an agency that will fit your needs and desire. Strive for it.
Remember an agency can get you infront of the clients, but the rest is up to you.