SAT and PSAT scoring help?

ok how in the world do i figure out my psat score...i got it today and i got like 20 different ways to do it and some ppl say it out of 1600 hundred other say out of 2400...by the way i got a 55 in critical reading a 45 in math and a 45 in writing skills...wats that come out too and could u explain it too...also wats the average score for a 10th grader?