Please read the course's FAQ sticky post here https://www.duolingo.com/comment/23799672.
If you are using a computer, read the Tips and notes section under the lessons - this is not available on the apps. Welcome to the course!
If you have a tabletI you can go into settings and click on language and input you should be able to activate the greek keyboard. Then when tou are typing click on the button that looks like a circle with line.