If you have money to blow, you COULD attend a "Modeling School or Academy".....but they are not always what they are cracked up to be. In fact...I would venture to say that MOST of the well known professional models have NOT attended a modeling school. The problem is that MANYYYY of them are nothing but a "front" for a photographer. Of course they will insist that all of your photos come from this ONE photographer or his AGENCY...so it's all about money, money, money. Most ad agencies won't ask the model if he or she was "schooled" by a professional Modeling School or not. It's all about the portfolio and experience in this job.
Experience is more important than school. It doesn't really make much difference if he or she was or not formally trained. I have gotten excellent results from the models I have used who have NEVER attended a Modeling School. If they are given 10 minutes of positive coaching and constructive criticism in a caring manner, they usually catch on well.
What IS important in the process is your ability to make a GREAT portfolio...that is....a decent albums of your best shots in a variety of setting. That includes bikini photos, head shots, formal wear, artistic shots, product promotions, you name it. The portfoilio must display the model in a variety of settings and backgrounds to display to a potential advertiser that she is versatile and capable of promotion the product or merchandise in the way that reflects best on the company. I have used many models who never show their face....so good looks don't ALWAYS carry a lot of weight either.
Age 12-13 is a good age to get going in THIS business.....many models are starting at around age 12, 13 or 14. I once created a clothing cataloh for children and tweens. None of the models were over age 16. With a little coaching they all performed like porofessionals and the catalog was a complete success.
I suggest that you find a photographer who is will can capable of helping you make a portfolio. Some photographers (such as new photographers or college students) may work with you on what is known as TFP...(Trade for Photos)...in other words.... you sign a Model Release allowing the photographer to use your photos for FREE and he supplies a great selection of photos for you to keep (for FREE) in your portfolio. One hand washes the other in this kind of agreement.
Also....your BEST choice of Modeling Agencies will be in the larger cities...New York, L.A. , San Francisco, Atlanta, Miami, etc.....but I don't necessarily suggest that you move, until you are better established. Also...don't be afraid to post a few photos HERE for us to see as well. Be sure to have an online presence at websites such as modelmayhem.com, onemodelplace.com or other such sites. Agencies DOOOO look at these sites from time to time to scout for new talent. There are probably some decent photgraphers in the YOUR area or some other nearby larger city.
Good Luck