How do I contact researchers for an internship?

I'm a junior in high school right now and I'm trying to get an internship at NIH. I'm just wondering if I should contact the scientists all at once or if I should contact one, wait for the answer, and then if the answer's no, contact another one? If I should contact all of them at once, what do I do if more than one researcher agrees to take me on? I'm not really sure how I should respond to the scientist that I don't choose to work with.