Shock of my life

My name is Jackie and I'm 18 years old and have accidently found out I'm adopted. I'm currently in my senior year and one day I went to the filing cabinet to get tax records to apply for financial aid. Instead, I found a file with my name on it, chockful of records with my name on it. I also found out that I may have a sister. As I looked through records I found a photo, Mia and Jackie, 1994 with a woman holding me, us. I'm presuming that I am adopted and have a twin somewhere. I don't know any other info, no mother, father or orginal last name. Just Jacqueline Hannah and a supposed sister, Mia. I'm scared to confront my parents, and I want to search a little on my own before confronting them. What should I do?