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.
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)
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)