Just say, “Hi, my name is _____, I’m 20 years old and looking for a shidduch, and I was given your name.”
Let the shadchan take it from there. She/he will probably want you to email a resume and to set up an appointment to meet in person. Or she may ask you over the phone to tell her/him about yourself and what you are looking for. Be prepared with a basic speech about yourself and what you are looking for.
I think it would be a good idea (if possible) to speak to someone first to get a good idea of what to expect when meeting a shadchan and to help you prepare what to say. Also, to help you get an idea of what you are looking for. If you have no one to speak to, don’t worry about it. But try to think if you have anyone to speak to. Maybe a teacher, or a neighbor, or a family friend, or a friend who is already in shidduchim.