The rule for preceding an acronym with “a” or “an” is similar to the rule for preceding a word with “a” or “an.”

If the first letter for an acronym makes a vowel sound, you would precede the acronym with “an.” Consonants that create a vowel sound include: f, h, l, m, n, r. s, and x.

It should also be noted that if a vowel letter that begins an acronym has a consonant sound, you should precede it with “a.” For example, “a U-turn.”
(Source: Grammar.com)