The law is interdisciplinary and branches out into numerous fields of law. While you do not need to know with 100 percent certainly which type of law you want to practice before you go to law school, it is important to explore the various legal specialties to confirm you want to practice law. Remember, when you graduate from law school with a J.D., you can practice any type of law (except for patent law which requires a technical undergraduate major). LSAC provides a summary of the most common fields of law.
Many books exist that describe these fields of law in much more detail. Penn State University Libraries/ Social Sciences Library can help you locate books and online resources to learn more about these fields.