The rule about the usage of a and an as indefinite articles is that an is used before a vocal sound.
- a warranty
- a user (because the u is pronounced like a w)
- a man
- an angel
- an information
When used before an acronym, the rule is still valid, but which article to use depends from how the acronym is pronounced (letter by letter, or like if it would be a word).
- an MP3
- an RPG
- an FBI agent
As example of acronyms that are pronounced as if they would be a word, I can think only to laser and radar, but those words (which are reported to be acronyms from my dictionary) are probably perceived as words; therefore, they are not good examples.