Om gam ganapataye namaha
A very loose translation might be:
Om = Salutations! Everyone wake up!
Gam = The secret power sound of Ganesh. It is his "seed syllable" or bija mantra.
Ganapataye = Another name of Ganesh, the breaker of obstacles.
Namaha = Yo! Ganesh! You da God!
The pronunciation is also pretty easy:
Om = ohm or aum
Gam = Somewhere between "gahm" and "gum". In some dialects it is "guhng".
Ganapataye = gah-nah-paht-ah-yeh
Namaha - nah-mah-ha