On a broad level, anthropology and sociology address similar questions/areas. Anthropology is the general study of humans; so while you’ll probably focus primarily on cultural anthropology (the study of human culture and cultural variation), you’ll probably also take classes in physical/biological anthropology (the study of the biology of humans and other primates), linguistic anthropology (human communication), and archaeology.
Sociology is the study of social relationships and interaction. Culture is a focus as well, but primarily for how it influences social interactions and relationships. Sociologists usually concern themselves with social stratification, social class (especially race, socioeconomic class, and gender), social mobility, and deviance, among other subject areas.
A major difference between the two are methods used. Cultural anthropologists use almost exclusive qualitative methods in their approach to answering questions. Ethnography is a key method of the field, but interviewing, participant-observation, naturalistic observation and other qualitative methods are commonly used by anthropologists. Sociologists use a mix of qualitative and quantitative methods. Some sociologists use ethnography; sociologists also use interviews, focus groups, case studies, social surveys (small and large-scale), longitudinal studies (studies conducted over time) and occasionally even experiments.
My guess is that sociology may be a (slightly) better fit based upon your interests. However, what I would recommend is that when you get to college, you sit in on the intro class for anthropology majors and the intro class for sociology majors. I had a similar struggle when choosing between sociology and psychology; I ultimately chose psychology after taking the intro class in both fields.
Also, many schools have majors that allow people to study the intersection between the environment and human behavior. Majors in (human) geography do this (such as [this one](BA in Geography | Geography, Environment & Society | College of Liberal Arts) at the University of Minnesota). So do environmental studies majors at many colleges (like [this one](http://www.es.ucsb.edu/academics/es-major) at UC-Santa Barbara). Many community and/or public health (sometimes also called health, culture, and society or something similar) programs also explore such issues, as environmental public health is one of the five main fields of public health.