A and an are articles that proceed and identify nouns. A and an can also be used to mean one of something. The choice of a or an before a noun is based on the phonetic sound of the first letter in the word, not the written letter.
A is used before all words that begin with consonants.
Example: a kite, a man, a tomato, a large apple
Exception: An is used before words that begin with an unsounded h.
Example: an honor, an honest mistake
An is used before all words that begin with vowels.
Example: an apple, an egg, an Italian, an onion, ...