indefinite article: a and an

1. We use the indefinite article, a/an, with count nouns when the hearer/reader does not know exactly which one we are referring to:

Police are searching for a 14 year-old girl.

2. We also use it to show the person or thing is one of a group:

She is a pupil at London Road School.

Police have been searching for a 14 year-old girl who has been missing since Friday. Jenny Brown, a pupil at London Road School, is described as 1.6 metres tall with short blonde hair. She was last seen wearing a blue jacket, a blue and white blouse and dark blue jeans and blue shoes. Anyone who has information should contact the local police on 0800349781.

3. We do not use an indefinite article with plural nouns and uncount nouns:

She was wearing blue shoes. (= plural noun)
She has short blonde hair. (= uncount noun)

Police have been searching for a 14 year-old girl who has been missing since Friday. Jenny Brown, a pupil at London Road School, is described as 1.6 metres tall with short blonde hair. She was last seen wearing a blue jacket, a blue and white blouse and dark blue jeans and blue shoes. Anyone who has information should contact the local police on 0800349781.

4. We use a/an to say what someone is or what job they do:

My brother is a doctor.
George is a student.

5. We use a/an with a singular noun to say something about all things of that kind:

A man needs friends. (= All men need friends)
A dog likes to eat meat. (= All dogs like to eat meat)

Source: http://learnenglish.britishcouncil.org/en/english-grammar/determiners-and-quantifiers/indefinite-article-and