Go to the Omniglot website of Vietnamese Language, Alphabet and Pronunciation.
Study Omniglot's list of letters in the Vietnamese alphabet, and learn how they are pronounced by reading English translations below each letter. Practice copying each letter and the diacritic marks.
Pay attention to the different diacritic marks. The first one is a level tone, the second one is a high-rising tone, the third is a low/falling tone, the fourth is a dipping-rising tone, the fifth is a high-rising glottalized tone, and the sixth is a low glottalized tone. The word "ma" using each of those respective tones would mean "ghost," "cheek," "but," "tomb," "horse" and "seedling."
Go to Seasite's Vietnamese Alphabet Table website.
Click on each letter, marked with different diacritic marks, to hear how it is pronounced.