As it is clear you are new to Duo here are some links to help get you started.
New user FAQ https://forum.duolingo.com/comment/1426103
The entire first skill is devoted to teaching the alphabet and the Tips ¬es of the first four skills contain material about the alphabet. [color=blue]Tips and notes[/color] can be found by hitting the lightbulb image at the start of the lesson.
In addition, these have been created by the Greek team for our learners: Accession each of these will bring you to other more specific links.
The Greek Language Forum: https://forum.duolingo.com/topic/936
HINTS ON HOW TO LEARN THE MODERN GREEK ALPHABET https://forum.duolingo.com/comment/23430853
HOW TO ADD THE GREEK ALPHABET TO YOUR KEYBOARD https://www.duolingo.com/comment/23430663
The Greek Alphabet: https://www.duolingo.com/comment/22424028
MODERN GREEK RESOURCES How to get the keyboard and much more.
If you have any questions please ask.