When you're a junior, you can take the PSAT exam (not all students are given this test, so check with your school).
If your score is high enough for your state's cut-off, then you become a National Merit Semi-Finalist - you find out around Sept of senior year.
If you make NMSF, then you fill out the forms, take the SAT, your school sends transcripts and writes a rec....if everything is ok, then you make finalist.
15,000 students make Finalist, but only 2500 are given a ONE TIME award of 2500 dollars. However, some schools give NMF scholarships if you attend that college.