Use the following words. 1- use "a" only with singular. 2- use "an" whith words that with a vowel sound.
According to the Oxford Dictionaries it is a determiner that introduces a noun phrase. a | an are considered indefiniten articles and they are used to introduce new concepts intro a discourse. Or a scheme.
What are then articles?
Topic principal
She is an honest woman. "Es una mujer honesta".
The boy eats an apple. "el niño se come una manzana".