The indefinite article a is used before words that do not start with a vowel sound; an is used before words starting with a vowel sound.  The rule is often misunderstood as saying that a is used before a word that does not start with a vowel, and an is used before a word that does.  However, pronunciation (sound), not spelling, influences the choice of the article.

Notably, several words begin with the vowel u but are pronounced with an initial consonant sound (often the "yuh" sound):  a union, a unicorn, a United States senator, a useful idea, a uniform.  Also a few words that begin with consonant h are preceded by an because the h sound is not aspirated (pronounced):  an honor, an honest man.

What of acronyms?  Again, we go how they are pronounced.  We say "an FBI agent" because we sound the letter F as "ef."  We say "a UN resolution" because we sound the letter U as "you."