Here is generic advice, but still applicable to Harvard. (I interviewed there but it wasn't a good fit for me.) Your major is not important. You have to take the science prereqs and do very well in all of them, as well as your other classes. It it merely coincidental that a lot of dental students are science majors, because by the time you have done your prereqs, you are close to a science major anyway. But there are plenty of non-science majors as well. So, the lesson is: major in whatever you will excel in. If that is science, then you will be like most people, but it doesn't matter either way as long as you have an awesome GPA.
You also need to have great DAT scores (there average is around 21, so shoot for at least that).
Research experience looks good on a resume for any school. I assume it is no different than Harvard.
Be prepared to spend a ton of money on tuition.
If you want to be a general dentist Harvard won't give you much of an advantage over other schools. If you want to go into academia or a specialty it will definitely help.
Good luck.
