Many people adhere to a belief that you use the article "a" before words that begin with consonants and "an" before words that begin with vowels. But that isn't the rule, and it's important to avoid this rookie mistake before turning over your manuscript to agents and editors.
The real rule is this: You use the article "a" before words that start with a consonant sound and "an" before words that start with a vowel sound. For example, He has a unique point of view on the subject and talked about it for an hour. The "u" in "unique" makes the "Y" sound—a consonant sound—therefore you use "a" as your article, while the "h" in "hour" sounds like it starts with "ow"—a vowel sound.