a = indefinite article + words starting with consonant.

• She has a flat in the centre.
• Can I have a glass of water, please?
• He is a doctor.

an = indefinite article + words starting with vowel.

• She always wears an old dress.
• Can I have an ice cream, please?
• I am reading an interesting book.

The indefinite article a or an is used with singular nouns to refer to a thing for the first time.

The = definite article.

• The flat is very nice.
• The ice cream is disgusting!
• The book is on my desk.

The definite article the is used with singular and plural nouns when both the speaker and the listener know the thing already.