sit down with them sometime when you can all focus. just tell them that you've been thinking about it for a long time and you'd really like one. if your grades are good, bring them up.if you do a fair amount of stuff, tell them that you run around a lot and would feel better if you could have one to call them to pick you up, or in case of an emergency. and maybe suggest that you'll do some extra chores in order to get one. :)
Sit down and talk about it with your parents. Organize data about why you want a phone, and the give good reasons for why you want it. Is it because your getting teased? Is it your feeling left out? Its helpful for you to write you reason down, or put it on a presentation.