How do you say goodafternoon in Myanmar?

Kaung Thaw (Good) Nga-Nay-Khin (afternoon/evening) Par (sentence ending particle).

There is no equivalent pronunciation for "Nga" in English. Try to pronounce to by saying 'na' with your nasal voice. If you cannot do that just say 'Na'-Nay-Khin quickly and no one will not probably notice it. "Th" in Thaw (Good) is pronounced as in 'Th' in That. The rest are pronounced as they are spelt.