The basic use of English articles is quite simple.

A & an are indefinite articles.
The is the definite article.

a/an vs the:
Use A/AN when you are selecting one of a group and THE when there is only one of something.

A LITTLE MORE ADVANCED

Now you know the basic idea, so let's explain more details about the article:

The Article

DEFINITE ARTICLE
The singular & plural
- the boy / the boys
INDEFINITE ARTICLE
A/An singular
A + consonant:
- a boy, a car
AN + vowel
- an apple, an ambulance
We don't use this article in the plural:
- I can see a boy and trees (not: a trees)

We use the definite article if everybody knows what we are talking about:
- the sun is yellow
- This is a family. The mother is Susan
We use the indefinite article when we don't know (or they don't know) what we are exactly talking about:
- I want a car (I'm not thinking of a particular car, this car is fine, or that car is fine, or your car is fine, I don't care)
- This is an apple (we didn't know this apple before)
That's why we usually use A the first time we talk about something, but later we use THE because now everybody knows exactly what particular thing we are talking about:
- This is an apple and this is an orange. Which do you prefer?
- I prefer the apple (now we know exactly what apple we are talking about)

We don't use the article in front of proper names:
- This is an apple / This is Tom (not: This is a Tom)
- I like the sun / I like New York (not: I like the New York)
but we can say: I like the city of New York
There are some exceptions. We can use the definite article in front of names of:
- rivers, seas: the Nile, the Mississippi, the Pacific, the Atlantic
- deserts, forests, gulfs, etc: the Sahara, the Black Forest, the Persian Gulf
In all these cases there is an article because the article goes with a common noun, present (1) or understood (2):
(1) common noun understood:
- the desert (this is not a proper noun)
- the Sahara desert ("the" still goes with "desert")
- the Sahara (the word "desert" is not present but it is understood)
(2) common noun present
- the forest
- the Black forest

We don't use the article when we are talking about something in general:
- I like flowers (flowers in general, all the flowers)
- I like the flowers in your garden (I'm talking about a particular group of flowers: the flowers in your garden)

We don't use the indefinite article (a/an) with words that have no plural:
- I have a book (1 book, 2 books)
- I have water (1 water, 2 waters)  (but we can say "the water": - Pass me the water, please)
These nouns are called "uncountable" and we will see them later.