I'm studying Japanese in school, and the correction pronunciation of the word is, as Krystal said, "mon-ga" or "mahn-ga."
The Japanese symbol ま "ma" is pronounced as a short "a" sound in English.
If "manga" were to be pronounced "main-ga", it would be spelled with め "me", in which the "e" is phonetic for the long "a" sound in English. And the roomaji would be spelled, "menga".
So in short, it's pronounced "mon-ga". =]