I understand your concern--many students were in your position, not too long ago! Finding a suitable mentor is a daunting, but incredibly rewarding task. Let me guide you through it:
Your concern is this: "...isn't it a bit preemptive to start emailing professors and looking into mentors? I feel like I would not be doing anything else but making a fool of myself if I ask for help understanding their research. But if I don't talk to a professor, there's nowhere to go from here."
It's understandable to feel this way; however, you may be surprised to know that most of the professors feel entirely OPPOSITE to that. They know that you're in high school, they know that, should they accept you into their lab, they will need to provide the resource and time to help you understand what their research is, develop your own project, etc. And guess what? They like helping you. People at the lab will expect that you'll need help understanding their work, and will gladly show you the ropes. No one will think that you're in "over your head" (even if you might feel that way sometimes
If you have such great access to Dartmouth and have 8-12 hours/wk to spare, I strongly suggest you start looking for a mentor at the university lab. You've already begun with researching your topic: that's very good. I advise you to start browsing the Dartmouth university research website, and make a list of 10-15 professors you'd like to work with. Read their websites to find out what sort of research they're doing, give a few cursory glances to their published papers, and think about how their research might potentially translate into a H.S. science project (but more on that later). Then, send them emails individually, briefly explaining your situation and interest in their lab. Ask them politely if you can meet with them sometime to talk about the research going on in their lab, and state that you're very interested in working with them. (You can also ask some friends/teachers how best to approach a professor).
Understandably, depending on their teaching load for the semester the professors may or may not accept you into their lab. It won't be a personal thing--they're just so dang busy! If you do get a response from someone willing to meet up, make sure you go back to their website and do some more reading.
Another way to find a mentor is to talk to your science teachers. Many times, H.S. teachers have friends who work at the local universities, and are more than happy to refer a student to them.
With all that said, give it a try, and see how things work out. Don't be disheartened if some decline, because it may just be that now's not a good time for them. Just thank them for their time and move on.
I was in your position too around the same time last year, so I'm glad to see that you're getting an early start!
Please don't hesitate to PM or post back if you have any more questions.