"The jacket of the boy."
Translation:Το μπουφάν του αγοριού.
You may want to read the comments in this discussion: https://www.duolingo.com/comment/17496123 I suggest doing an image search, it's very reliable.
What keyboard layout do you have? On a standard QWERTY, accents are added by hitting first the key next to L, then the vowel (; or : + vowel): ;+α=ά.
Των is the plural masculine article in genitive, whereas του is the singualr masculine article in genitive. So if we were to use των, we'd say "Το μπουφάν των αγοριών" (The boys' jacket).