There are two types of articles indefinite 'a' and 'an' or definite 'the'.
A and an are the indefinite articles. They refer to something not specifically known to the person you are communicating with. A and an are used before nouns that introduce something or someone you have not mentioned before, they are also used when talking about your profession ( I'm a teacher)
You use the when you know that the listener knows or can work out what particular person/thing you are talking about.
You should also use the when you have already mentioned the thing you are talking about.
We use the to talk about geographical points on the globe (the North Pole, the equator)
We use the to talk about rivers, oceans and seas(the Nile, the Pacific, the English channel)
We also use the before certain nouns when we know there is only one of a particular thing(the rain, the sun, the wind, the world, the earth, the sky,etc. )
However if you want to describe a particular instance of these you should use a/an.
(I could hear the wind." / "There's a cold wind blowing)
The is also used to say that a particular person or thing being mentioned is the best, most famous, etc. In this use, 'the' is usually given strong pronunciation
(You don't mean you met the Tony Blair, do you?)