How do I find out who my biological parents are when I turn 18?
First, if you are really lucky your adoptive parents have this information, and will give it to you if you ask. They may be waiting for you to ask, so please do so even if you think it may hurt their feelings. You can explain that it does not mean you don't love them, but that everyone naturally wants to know about their birthparents--for medical information, if nothing else. I do realize this may not be realistic in your family so below are other options to find your birthparents.
In some states you can ask for your original birth certificate which will list the names of your birth parents. Many states, however, still do not allow adoptees to have this information. If so, you should contact the adoption agency or adoption attorney who placed you, and see if they have any information they can provide about your birthparents (other than non-identifying information). If your birthparents are looking for you they may have left a letter at the agency for you. If this fails, there are many "search angels" on the Internet who will help adoptees search for their birthparents. These are free services.