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 U is pronounced like a Y)
- A one-way (because O 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
- A GPS device
- A NASA employee (because NASA is pronounced /ˈnæsə/)