Reddit mentions of Core Concepts in Cultural Anthropology

Do you have access to classes at your local community college? There might be an intro-level cultural anthropology class that you could enroll in there.
The reading list that BentNotNroken linked to looks excellent, but if I was a high school student interested in learning about the field, I would find it very overwhelming! If you'd like a shorter list, here are the books I read in my first intro to anthropology class:

Core Concepts in Cultural Anthropology by Lavenda & Schultz

This is a super basic primer that will introduce you to a lot of the basic concepts and terms of cultural anthropology. It's very easy to read, and I still come back to it often if I can't think of a well worded definition/explanation of a term.

Coming of Age in Samoa by Margaret Mead

This is sort of a classic example of an ethnography. If you don't choose to read this now, you will absolutely read it at some point once you start taking anthropology classes. It's not too dry, interesting to read, and will give you a good idea of what anthropologists study and do.

White Saris and Sweet Mangoes by Sarah Lamb

This is another book that you will likely read at some point if you take cultural anthropology classes! Like the last book, it's just a basic ethnography that will help you understand the point of anthropology.

Coming of Age in Second Life by Tom Boellstorff

This ethnography is still one of my favorites! It basically takes the concept of ethnography that is used in the previous two books and applies it to an online community rather than a real-life one. I found it to be a fresh and modern example of the possible applications of anthropology. If you find the previous two books boring, you will probably enjoy this one more, because it's on a topic that you might find more interesting, since you participate in online communities yourself!

The books I listed are focused mostly on cultural anthropology, which is one of the four basic subfields of anthropology as a whole. The other three subfields are explained here if you are curious. If you are interested in one of those, perhaps someone else here can suggest some basic books to start with! If you are interested in archaeology, I can provide you with some readings, but my studies focus more on classical archaeology, which some would argue has little to do with anthropology.

Good luck with your studies - if you have any other questions feel free to ask!