I too was in your position a number of years ago. I basically already knew that I had it, as did all of my peers, but I didn't know how to approach my parents about it. I found that it was easier for me to write it in a letter than to address them in person.
So...I took a few days, or maybe it was longer, and wrote a letter to them that outlined what AD/HD is and some of the symptoms. I also included symptoms that I display w/ some examples. I concluded it saying I thought it would be beneficial for me to be evaluated so that I could figure out what I did/did not have so that I could go about helping myself in the best way possible.
Another option would be to talk to a teacher that you feel comfortable with or a school counselor, or any other adult whom you feel you can talk to. They can probably help direct you along the "right path." I do know that a number of students go to school counselors because they suspect that they may have AD/HD; and all of the students that I've known to do this have been very pleased and comfortable with how it went. The counselors that I've talked with about it have also expressed a lot of interest in helping those students.
Good luck and keep us posted about what you end up doing and how it goes! Feel free to post any other concerns or you can PM (personal message) me if you want! (With questions or concerns or anything!)