It should be 'Είναι ένα ποτήρι' since 'This - Αυτό' is not used
"Αυτό είναι ποτήρι" should be correct, no?
Yes, it is.
How do I type in Greek? I get so far with translation but can't complete exercise as I don't know how to get the Greek alphabet.
This page has a lot of information.
MODERN GREEK RESOURCES How to get the keyboard and much more.