Use a before a consonant sound:
a historical guide, a onetime king, a U-turn.
Use an before a vowel sound:
an onion, an 11-year-old child, an L-shaped structure.
Before abbreviations, including acronyms, use a or an according to the pronunciation:
an AWOL soldier, a GI.
Words with a voiced h are preceded by a (e.g., a historical guide); an unvoiced h takes an:
heir, herb, honest, honor, hour.
Most of the derivatives of these five also take an except for the derivatives of herb, most of which take a:
an herb, a herbivore.
After a possessive, omit initial A or An of a title:
Dickens's Tale of Two Cities, Ambrose Bierce's Occurrence at Owl Creek Bridge.