The SAT tests your writing skills in two ways:
Use our SAT essay section to get clear advice on how to handle the essay. We have a topic list from which you can select some essays to practice. You can get your teacher to correct your essays or use our essay evaluation grid to help you estimate your own score.
The grammar questions are of three types:
Our 10 free SAT grammar practice tests give you practice on all the types of question you will encounter on the test. You can get explanations for any questions that you get wrong.
For further discussion of typical grammar problems you can refer to a book such as the Kaplan SAT Writing Workbook. This book also has a good section on how to improve your essay writing.