A or An

Use “a” when the first letter of the word following has the sound of a consonant. Keep in mind that some vowels sound like consonants when they’re sounded out as individual letters.

Examples:
a frog
a hospital
a universe

Use “an” when the first letter of the word following has the sound of a vowel. Remember that some consonants sound like vowels when they’re spoken as individual letters.

Examples:
an FBI case (“F” is pronounced “ef” here)
an honesty (“H” is silent here)
an unreal idea

Deciding whether to use “a” or “an” before abbreviations can be trickier. The abbreviation for “Frequently Asked Questions (FAQ)” causes confusion because it can be pronounced as a word (fak), or one letter at a time (F-A-Q). Using the guidelines above, one would say "a FAQ" when it is pronounced as one word, and "an FAQ" when it is pronounced one letter at a time.

Examples:
an FBI agent (“F” is pronounced “ef” here)
a FICA tax decrease
an HMO plan (“H” is pronounced “aych” here)
a HUD program
an NAACP convention (“N” is pronounced “en” here)
a NASA study

bibliografia: The Blue Book of grammar and punctuation by Jane Strauss