There are several labs at my medical school that take on High School students for summer research. One of them is my lab, and I've worked with a few of the students, so I may be able to answer some of your questions. 🙂
As for finding research options, it's really just a matter of finding a lab that enjoys teaching students. If you live close to a medical school, it might be a good idea to see if they have a mentorship program established. A mentor could point you in the right direction for finding a lab, and they may even be able to answer some of your technical questions down the line. If that isn't an option, but you live close to a university/research center, just go to their web page and browse investigators. Find a few research areas that seem interesting, email those investigators, and ask if they would consider taking on a high school student.
As for what you will do, this will vary greatly by lab. While you probably will not be working with anything dangerous for safety/legal reasons (e.g. handling animals, working with strong chemicals, etc), you could certainly run assays, help with big experiments, and analyze data. You will probably understand what you are doing in broad terms (you may understand what a color change in an assay tells you, but not why it tells you that), and for a high school student, that is alright.
Just make sure that you can perform your experiments well and don't cause trouble for anyone else in the lab (don't use things without permission, etc), and you should be just fine. If you have questions, ask a graduate student or an older undergrad for help before going to the investigator or postdocs - and always ask for help when you need it.
Good luck.