Hi, Andrew—Technically, an acronym is a word whose letters each stand for another word. For example, "SCUBA" stands for "Self-Contained Underwater Breathing Apparatus," "WASP" for "White Anglo-Saxon Protestant," and AIDS for "Acquired Immuno-Deficiency Syndrom."
We use articles with acronyms like the above just as we would with ordinary words like "apple," "car," etc. However, I assume that you mean to ask about initialisms like USA, FBI, CIA, etc. With these, article usage generally varies based on phonetics: viz., an FBI (an "eff . . .") agent, but a CIA (a "sea . . .") agent.
Occasionally, however, an initialism is pronounced as the words for which its letters stand rather than as the letters themselves. In that case, it is the expanded version that determines which indefinite article is to be used. Thus, some linguists write "an NP," and others write "a NP":
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