The most important credential considered by law school selection committees are the results of your Law School Admissions Test (LSAT). The LSAT is a national exam written in similar style to the verbal and essay sections of the SAT.
The next most important credential is your college transcript and especially your cumulative grade point average.
Not as important, but still meaningful in the final decision are letters of recommendation (best if written by faculty members).
Depending on the law school, other factors carry different weightings like ethnic background, gender, college activities, special interests, distinctive abilities, a personal statement, law related experience, and so forth.