The best solution is to simply download a Greek keyboard. I'm sure that you can find an app on whichever app store your computer uses. If you also use a phone, adding languages will be under System->Language and Input -- then you can usually switch between keyboards with the spacebar.
If you are on the website, check the Tips and notes section under the lessons of the first skill. you can also check the the course's FAQ post here https://www.duolingo.com/comment/23799672. If you are on the app, use a browser to go to the website! :) The Tips and notes (available for most(?) skills) are too good to miss but they are not available on the app.
If you have trouble accessing either the Tips or the FAQs post, google "how to add greek keyboard on ___" and fill in with the type of device you're using.