A. The article A is used before singular, countable nouns which begin with consonant sounds.Examples:
1. He is a teacher.
2. She doesn't own a car.

B. The article AN is used before singular, countable nouns which begin with vowel sounds.Examples:
1. He is an actor.
2. She didn't get an invitation.

C. In English, some nouns are considered uncountable such as: information, air, advice, salt and fun. We do not use A(AN) with these uncountable nouns. Examples:
1. She gives a good advice. Not Correct
2. She gives good advice. Correct