If you've actually taken the LSAT, the LSAC should have provided you both your score and your percentile for that test. You can get this information either over the phone (if you called in for your score), or on the score sheet that they send you in the mail. If you've just taken a practice test, a score vs percentile table should have been provided at the end of the test section. If you're just trying to get a general idea of typical score vs percentile, you can check out an LSAT practice test book at Barnes & Noble, Borders, etc. These books have several practice tests with score/percentile tables for each test.
I'm not sure if I've addressed your question sufficiently, so let me know if you need any more detail.