What to do to tell my parents i'm bulimic, i'm 17 years old?

Ask to talk w/ them: You might start by saying that this is hard. That you've been worried about the way you eat sometimes and that it's feeling like you need help to stay healthy. When you describe what you mean, be as specific as feels right. Bottom line is that you'd like to get expert help to avoid getting too bad. You might tell them that you are not blaming, just worried and want to feel better. Hope this helps.