The first question you would have to answer is whether you are having any experiences that would lead you to believe that you are able to communicate with those who have passed over. Most who are able to do this are born with it, and do not come into it without having experiences that they cannot explain. Whether it is seeing, hearing or feeling spirits. If you feel as though you have had these experiences, seek out a metaphysical book store in your town and see if they have any workshops that you could go to. Also, if there are any Spiritualist churches in your area. Since you are 14, you would have to have your parent(s) take you. One does not really *train* to be a medium, you just are. It isn't a vocation like training to be a nurse or a computer technician! You either have it, and are able to build on it, or you don't.
You could try finding your nearest spiritualist church they have regular meetings and joining a spiritual circle will certainly benefit you. Good luck.
Most have a natural talent,don't know of any place in which you get professional help with this,if you know a professional medium perhaps being a apprentice could be helpful,working around or with a medium could prove to be helpful....good luck