As a freshman, sophomore or first-semester junior, there is nothing in particular you need to be doing to prepare for law school other than take your classes, do well and be involved in high-quality on- or off-campus activities.
In the spring semester of your junior year or fall semester of your senior year, come to a law school application workshop. These occur within the first two weeks of the start of classes each semester. The workshops cover strategies for applying to law school, putting together your application, personal statement, recommendation letters, and taking the LSAT.
Subscribe to the prelaw advising listserv to learn about the workshops, visits by law school admissions personnel and other law-related events on campus.