Well, first off, you are going to need you parents permission, because any contract that you may have with any agency or client will have to be signed by them until you are 18. Generally, girls may be physically ready to start modeling high fashion at the age of 15. Prior to that there are many jobs in commercial print work and lifestyle work that anyone of any age may do.The first step is to find an agency that will represent you. You will have to do research to know which agencies will represent minors. Many will avoid it as much as possible because there are too many scheduling conflicts as well as legal issues. Thats not to say they will never work with someone under 18 but there would have to be some serious potential.Once you can find an agency, then you would have to get tested just like any other model. Depending on where you live and work, this may set you or your parents back several hundred dollars. There are opportunities to do free tests, but that is usually with new photographers who are trying to build a book for themselves and there is no guarantee that the photos would be worth your time.Once you have a decent portfolio, then you would be able to sent to Go Sees, in which you can meet with various clients to show you book or perhaps to show them your walk if they have an interest in you doing runway. If they hire you as a model, they will contact your agency and you will be on you way to a modeling career.