If you're on a computer (PC/Mac), try the key next to the L key, where the semicolon and colon are on a US PC keyboard.
Nothing happens, but if you then type a vowel letter next, you will get the accented vowel ά έ ή ί ό ύ ώ.
And to get the diaeresis (two dots), use Shift + that key, then the vowel letter: ϊ ϋ
For the (rare) combination of the two, try Shift+W, then the vowel: ΐ ΰ.
(Just W should give you a final sigma "ς", and just Q a Greek question mark ";")
Hello! Please read the sticky post MODERN GREEK RESOURCES How to get the keyboard and much more. :)