If you can get into a top college and you do well there, it will help you get into the most selective law schools. For example, according to this site, at Harvard Law, "Students from elite private universities, especially the Ivy League, Stanford, Duke, and Northwestern (among others), do seem to predominate."
The most important factors law school admissions committee consider are your grades and your LSAT scores. As a reference, the 25th to 75th percentile ranges for admission to the class of 2014 at Yale Law were 3.83-3.96 (GPA) and 170-177 (LSAT)
In law school admissions, your grades in college are a very important element. Law school admissions officers know from experience which departments at the top colleges have strong reputations and which courses have high and low curves. According to the Harvard Law School Admissions Office, academic success is important but other qualities that promote vitality, diversity, and excellence in the student body are also valued. As stated on its website, “The committee uses no computational methods for making decisions and no ‘cut-offs’ below which a candidate will not be considered. Each year we admit applicants who believed they didn't have a chance.” You don't have to fit a certain mold to fit in at a top law school.
The LSAT is required for admission to all American Bar Association-approved law schools, and is given 4 times a year. Test sites can be filled quickly, especially around big cities, so it's best to register several months in advance of a test date. The optimal time to take the exam is June of the year you apply, but if you take the test in late September/early October that still allows you to see your LSAT score before applying in November.
Law school admissions committees also consider extracurricular activities when reviewing applications. Things that law schools look for are leadership experience, work experience, research and publications, community activities and public service. However, YOU CANNOT sacrifice grades for extracurricular activities.