Step Two to being together?

My boyfriend [now fiance] proposed to me this weekend. I'm seventeen and he's eighteen. I'm not looking for people to criticize me saying we're too young for marriage, I'm just looking for some opinions on how to tell our parents that this is what we want. We already have our lives planned out with a few backup plans just in case, but they don't coincide with the plans our parents thought we were going through with. How do we tell them we've changed our minds?