You can find out more about it here: http://www.collegeboard.com/student/testing/psat/about.html .

The PSAT qualifies you as a “National Merit Semifinalist”, and then you have fill out this packet (they ask about your transcript, GPA, extracurriculars, you write a personal essay and get a recommendation letter) and take the real SAT and submit your scores to NMSC. You can see the steps to becoming a National Merit Scholar here: http://www.nationalmerit.org/nmsp.php .

The score that you have to get on the PSAT to qualify varies from year to year and from state to state. Look here to see about what you need to aim for in your state: http://talk.collegeconfidential.com/showpost.php?p=2993607&postcount=662 .

Good luck!