If anyone knows anything about PSAT/NMSQT please help?

if you have been a national merit scholar or know someone who has, or know anything about it do you know what you have to get in order to qualfiy im a junior now i will be taking it in october. I am not really sure how to prepare for it. What should I do? I bought a book. Last year my reading was an index score of 73 I think however my writing and math were somewher in the 50's so i need to get those 2 up, how do i prepare for this?