Practice SAT - Critical Reading: 410 Math: 670 Writing: 550 (Essay: 8/12) *No Prep at All*?

How can I improve on Critical Reading and Writing????? For the sentence completion, I'm bad at that because I don't know much of the vocab yet. For the passages, they get me bored and I do really bad on them. Any tips for each of these? Also, what can I do to improve the Writing section???