While good grades are essential, you want to present yourself as a well-rounded candidate, so plan to get involved in some extracurricular activities.
Get involved right away. Admissions committees will want to know how long you have been involved in each activity and what meaningful contributions you've made. If you wait until senior year to join a group, it may be perceived as a move you're making purely to increase your chances of acceptance rather than an involvement that sprang from a genuine passion or interest. Limit the number of groups you join. The depth of your involvement is much more important than the number of activities in which you are involved. 
Explore all of your options--affinity clubs, intramural sports, service organizations--then commit to two or three that you really connect with. Keep track of your hours of involvement because the applications often ask you to specify how many hours you spent on each activity per week.
Consider larger, more established organizations. Many prestigious nonprofits, like UNICEF and Habitat for Humanity, have college chapters on campuses all over the country. Take on a role in student government or on a school publication (newspaper or journal). These are highly valued by law school admission's committees.  Secure a leadership position. Whatever organization you choose to become involved in, be sure to step up and take a leadership role. It's essential that you be seen as someone who can take charge, manage multiple responsibilities and people and achieve results. Again, don't just pick something because it sounds good. Get involved and get passionate about something because you can, as these areas of interest can also be considered by law firms as you're looking for a job later.