Guidelines for using the Indefinite Article
A is used before nouns that begin with a consonant.
A boat. A car.
An is used before nouns that begin with a vowel or a vowel sound.
Vowels: a, e, i, o, u, y
Compare an hour, a house, a horse.
The indefinite article a/an is used with singular count nouns when the speaker is making a generalization.
I saw a cat in your trash can.
An apple is red.
A computer is expensive.
Do not use an article with plural count nouns when making generalization.
Cats are not usually noisy.
Apples are red.
Computers are expensive.
Do not us an article with non-count nouns when making generalizations.
I like coffee.
Milk is good for you.
Having money is good.
For plural count nouns and non-count nouns use some, a lot of, or a little.
There are some cats in the backyard.
I bought a lot of apples yesterday.
I would like a little cream in my coffee, please.
The indefinite article a/an is used with singular count nouns when the speaker is talking about one thing (or person) that is not specific.
I saw a child in the store.
The noun child is not specific. It doesn't say whose child. It could be any child, it could be my child, your child, or a neighbor's child, we don't know anything except it was a child.
On the second mention of a noun the definite article the is used.
The child was crying.
Now the listener knows which child the speaker is talking about: the child in the store.
Not the child at home, not a child at the movie, but the child at the store.