Use of 'A'
1. In the sense of one
e.g a one-man show, a one rupee note.
since 'one begins with sound of 'w'

2. Before words beginning with consonant sound
e.g a boy, a dog,a goat,a kite etc.

3. With Vowel letters having Consonant value
e.g a university, a European language

4. In exclamatory expression before singular countable noun
e.g What a pretty girl!

5. With certain expressions of quantity
e.g a lots of, a dozen, a couple

6. With a person's name to indicate the person is perhaps unknown to the person addressed.
e.g. A Mr.ROY is at the door