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 MPEG
- 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.