American law schools require applicants to take the Law School Admission Test (LSAT), which is prepared by and administered by the Law School Admission Council (LSAC). Information is available at www.lsac.org.
The LSAT is not a test of your knowledge; rather it is a test of critical thinking ability, reading comprehension, and writing. You may study and prepare for the exam by yourself, using books and practice tests that are available online or in stores. Alternatively, you may take LSAT preparation courses that are offered by commercial companies.
To apply to law school, you will submit an application, letters of recommendation (or electronic evaluations), a Personal Statement, a vita, all of your academic transcripts, and your LSAT. You do not have to prepare individual submissions for each law school you are applying to, though. Law schools require applicants to use the Law School Admission Council as the portal for application material.
Thus, even as early as your freshman undergraduate year, you should go online to www.lsac.org and create an account for yourself. By doing so, you will discover a wealth of information available through LSAC on law schools and on the application process. Then, when you are a junior or senior, you will need to pay a fee to the LSAC and start building your application materials.
Only you can determine that--but to do so, you should do some serious research--and perhaps even visit those schools you are most interested in. The pre-law advisor has materials from many law schools, available for your perusal. The LSAC and the American Bar Association (ABA) also publish an annual Official Guide to aba-Approved Law Schools, available in the pre-law advisor's office. Additionally, the LSAC's web site has extensive information on each law school and links to individual schools at http://www.lsac.org/JD/choose/law-school-links.asp.