You have already seen a, the Greek equivalent of our English a. Greek also has vowels that correspond to the other English vowels: e, i, o, u.
E, e is pronounced like the e of let.
I, i sounds like the i in pin or machine.
O, o is like the o in hot. Notice that this sounds very much like a. The only detectable distinction is that a is an ah sound, while o is an aw sound.
U, u should actually be pronounced like the German u in brüder, but the u in turn is a close enough equivalent for most people.