Use:

The article “the” is used for stuff or other nouns which you know. You only can use the indefinite article “a“, when the word after the indefinite artcle begins with a consonant for example b. The indefinite article “an” can only be used, if the noun or adjective after the article begins with a vowel for example a or it sounds like a vowel. The indefinite article can be used for stuff or nouns which you don’t know how it looks like. You can’t use the article for names for example “God”.

Example sentence:

(in-)definite article:

Erdogan is a politician of Turkey.

An hour has 60 minutes.

Definite Article:

Putin plays with Obama the game chess.

Merkel is a Dortmund-Fan, but the finance minister Schäuble is a Bayern-Fan.

Without an article:

Life in London is very expensive. (not: The life in London is very expensive.)