How is the SAT graded?

How is the SAT graded?
i took it yesterday and:
i left about 7-8 blanks in total from all 10 sections
i realized i got a math section question wrong after having thought it through after
and i didn’t get to finish the last sentence of my essay which had 3 paragraphs and took both pages given to us.
2 of the longer narratives i didn’t comprehend much and looked at the questions only to have to reread them again and answer (like: in lines 42-44… i had to reread)
i feel good about my score though, unless i get a 1900 i probably won’t take it again (now if i were to get a 2000, i will stick with that*) and on some sections i felt i had enough time, others not enough.

*the thing is, a first generation Mexican college student with a 1900+ can pretty much get me anywhere, especially since i got a 3.00 last year and this year i am planning to get a 4.3ish (shows improvement).
my sister (while she got A’s in basically everything) got like a 1900 or more and she got accepted in USC, UC Berkeley, Standford, and UCSD and got wait listed for other top schools which i forgot the names of. but she is way smarter than me, the SAT was most likely harder 5 years ago than it is now.
i felt like some of the questions were easy, and my SAT review book had much harder problems (especially math). so i basically studied until 12 am last night for nothing (the only thing that did help was the -b/2a thing for a graph).

and one of my friends said that if you get a good grade on the essay, you can omit or get a couple questions wrong on the critical reading section. and that missing easy questions hurts you more than missing harder questions. but then again he got a 2000 his first time

Your friend is not exactly correct. The Essay is part of the English/Writing section, not the Critical Reading Section. So, it can only help your English Grammar and Vocabular multiple choice score.

How it’s scored:
Essay – for a perfect score:
An essay in this category demonstrates clear and consistent mastery, although it may have a few minor errors. A typical essay:
-Effectively and insightfully develops a point of view on the issue and demonstrates outstanding critical thinking, using clearly appropriate examples, reasons and other evidence to support its position
-Is well organized and clearly focused, demonstrating clear coherence and smooth progression of ideas
-Exhibits skillful use of language, using a varied, accurate and apt vocabulary
Demonstrates meaningful variety in sentence structure
-Is free of most errors in grammar, usage and mechanics

Critical Reading –
Total number of questions – 67
Take the number that you have correct, and then deduct 1/4 point for each incorrect answer. Then, translate your score to an 800 scale.
Total Questions 67
Correct Answers 60
Skipped Answers 2
Incorrect Answers 5
Deduction 1.25 (1/4 point for each of the incorrect answers)
CR Core Score: 58.75
58.75 x 800 = 35250 / 67 = 526.12 (530 would be the score)

Math is scored the same, however, based upon only 54 questions.

English/Writing – The Essay represents 1/3 of your score, so, a 12 on the Essay represents 270 points. A perfect score on the multiple choice is a 530 (together, they add up to 800)
Total Multiple choice questions 50
Correct: 45
Skipped: 2
Incorrect: 3
Deduction: .75
English m/c Total: 44.25
44.25 x 530 = 23452.5 / 50 = 469.05
Essay Score: 10 out of 12
10 x 270 – 2700 / 12 = 225
469.05 + 225 = 694.05 So, the Total English/Writing score would be 690

Hope this helped