When you create a new form, look for the gear icon in the upper right hand corner. Click it. This opens the settings for the Form. There are three tabs - General, Presentation, and Quiz. Click Quiz and you will see this:
Click the switch to "make this a quiz" and then select any of the other options that you want. Make sure to look at the choices in the other tabs (General and Presentation) too so you can collect usernames if you want or restrict students to one response (one chance at the quiz) or allow many responses.
Click the plus sign to add questions. In order to have the questions operate as a quiz, you will need to choose one of the objective question types - multiple choice, checkboxes, or dropdown. These are the only ones that will allow you to mark an answer for the key.
Click on the words ANSWER KEY to select the correct answer for the key. When you do this, you will also have the chance to ADD ANSWER FEEDBACK that students will see after they select their answer. The feedback can be text or a link they can explore.
When students take the quiz, they will see the typical message after submission (this message is customizable, by the way) with a new addition: View your score. Click this to see how you did on the quiz.
After clicking View your score, students will see a score in purple in the upper right and each question, color-coded red or green for right or wrong. They will also see the correct answers.
Teachers can see the results, individually or for the group, by clicking the word RESPONSES near the top of the Form in edit mode. When you click RESPONSES, you can also click the green spreadsheet icon to create a spreadsheet of the responses.