All of the questions on the ACT are multiple-choice questions. You'll have four answers to choose from, except in the Mathematics Test, where you'll have five. On odd-numbered questions, the answers are labeled (A) through (D); on even-numbered questions, they're labeled (F) through (J). This labeling system has the advantage of helping you keep track of which question you are working on. After you pick the best answer, you'll mark your choice on the separate answer sheet by using a pencil to darken the appropriate oval next to the number for that question. This produces an answer sheet that can be scored a machine.
When you get the ACT booklet, you'll also get the answer sheet. 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. It'll be too sad if you knew the correct answer but didn't mark it right.
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.