Japanese has five basic vowels, usually referred to as short vowels, which are romanized as a, i, u, e, and o. These are pronounced approximately like the "a" in "father," the "ee" in "feet," the "oo" in "mood," the "e" in "met," and the "o" in "fort," respectively.
"Manga" is a Japanese word that means "comic book," so it should be pronounced the Japanese way -- which would be "mahn-gah."
Say it whatever way you want though; I say it the way I described above (sounds nicer). XD The word always makes me think of "Magna Carta."