I am taking the SAT in June, and I want to get a get a 1900+. I have little preparation, and my first SAT score was a 1540.
What are some tips?
Writing: What kind of grammar rules should I study? Where?
Critical Reading: I bought Direct Hits and I have been reading that and memorizing terms. And I am realizing if I actually get into ANY passage I am reading, I am able to do well on the questions. I typically lose interest and passively read it.
Math: I am pretty solid at. I will probably just do practice tests on top of practice tests.
Oh, and I have the Blue Book so I will do full practice tests there.
And for the ACT, I have the red book. Also taking this test in June. Never took it before so I am kind of nervous!
Writing: Subject-verb agreement, idiomatic expressions (i.e., different from vs different than), correct use of modifiers, knowing how appositives work
I’m a freshmen so I just started reading up on the SAT. I thought this ebook from crimson scholars was pretty good for basic tips like guessing. They say you can get 100 extra points, but i don’t believe that. The website is kinda annoying so I put it in my dropbox. Hope it helps! 
https://www.dropbox.com/s/jb3yi3ayan20h6q/100%20Point%20SAT%20Advantage%20-%20Crimson%20Scholars%20-%20Free%20eBook.pdf?dl=0
The best book that shows the grammar rules of the writing section is the ultimate guide of the SAT grammar by Erica Meltzer, but as #2 person said those are the basic grammar rules. You will see the grammar rules by studying the test.