LSAT stands for Law School Admission Test. All North American law schools require that you write this test before you apply for admission. The LSAT is run by a private testing company, not by any particular school, and tests are offered on a quarterly basis in cities across the continent. If I recall correctly, the same exact test is written by thousands of people across Canada and the US on the same weekend. Your score is not a percentage, it's a weighted score. In other words, the result you get is a statement of how you ranked compared to the thousands of other people who wrote the test. If you ranked in the 50th percentile, for example, you did as well as half the people that wrote the test. If you ranked in the 80th percentile, you did better than 80% of the people that wrote the test.

At least one company that I'm aware of sells study guides and actual past LSAT exams that you can test yourself on; you can find these sorts of study guides at places like Chapters and your local university book store.