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To refer to something (that is singular and countable) for the first time in a conversation or piece of text:
She's got a dog and a cat.
Do you want to see a film tonight?

With job titles, professions or to explain what people do:
She's an artist.
Alice is a nurse.
Helen's a student.
He's a house-husband.

Note: He's an unemployed.       He's unemployed.

With expressions of quantity:
a pair of, a few, a little, a piece of, a bit of, a hundred, a thousand, a million, an hour, a day, a week

To refer to something (that is singular or plural, countable or uncountable) when both the writer and reader, or speaker and listener, know which thing is being referred to:
She's got a dog and a cat. The dog is friendly but the cat always scratches me.
Do you want to see a film tonight? Yes, let's see the new Harry Potter one.

With rivers, seas and mountain ranges:
the Thames , the Atlantic , the Alps

Where there is only one of these things or people:
the White House, the moon, the UN, the EU, the president

With superlative adjectives:
She's the best in the class at English.
They're the fastest team in the race.

To refer to something (that is plural or uncountable) when talking about things in general:
France makes very good cheese.
She hates that kind of music.
That shop sells fantastic cakes, sweets and chocolates.

With streets, cities, countries and mountains:
Fifth Avenue, Beijing, India, Mount Fuji

With some places and some modes of transport:
to/in bed, at home, to/at work, at/to school/college/university, by bus/train/car, on foot

To put someone on hold (used in business telephoning): To ask a phone caller to wait (usually music is played while s/he is waiting).