When you get the SAT booklet, you'll also get a separate sheet on which you'll mark your answers. For multiple-choice questions, you'll see sets of answer ovals labeled from A to D or E. Remember, your marked answer sheets will be read by machines. The machine doesn't think. It calculates your scores by what your marks look like, not by what you really meant.
Fill in your chosen ovals correctly, completely and boldly as required, so there can be no mistake about which one you chose. If you knew the correct answer but didn't mark it right, that would be too sad.
For the student-produced responses, you'll still be filling in ovals, but you'll write in the actual numerical answer. You'll read detailed instructions later on when you practice at this Web site.
You should develop a good habit of regularly checking the number of the question and the number on the answer sheet every few questions. Check them carefully every time you skip a question. If you knew the correct answers but filled them in wrong places, that would be even sadder.