How to convince my parents to let me move out (PLEASE READ!)?

I am currently 17 and live with my biological mother and step father... by the beginning of the summer of 2012 I will be 18. I want to move in with my biological father while I attend college in Montana and so I can see the girl I have loved for 5 years. Everytime I want to tell my parents I would like to move in with my bio father I get scared that they will be angry and it's hard for me to want to leave them... I only plan to live with my bio father through college and med school then get a house of my own... I like everything about the idea of moving back to Montana from seeing the girl I love to the view to the college choices and to the people.... I just cant bring myself to tell my mother and step fater.... what should I do?