A student's performance in undergraduate school is considered a good indicator of law school success, so law school admissions committees look very closely at your GPA. They also look at coursework: Straight As in basket-weaving and history of modern dance aren't as impressive as the same grades in honors international policy or philosophy.
Admissions committees expect good grades in your junior and senior years, plus increasing advanced coursework. In fact, they like to see grades that improve throughout your college career, indicating maturity and intellectual curiosity.
A pre-law program is usually not a prerequisite for admission to law school, but your record should demonstrate a commitment to careful study, analytical thought, and written and verbal communications. Common majors for law school applicants include literature, history, philosophy, and political science, since these subjects help develop the ability to think critically and analytically. Writing skills are especially important in law school, so a minor in English can help you prepare for the rigors of law school -- and it will look good on your record.
If you have your heart set on attending a particular law school, see what undergraduate schools are represented among those currently attending. You may see a pattern: A highly rated school like Harvard or Yale accepts a large number of students from Ivy League schools; top regional schools may also have "feeder" liberal arts colleges.