A proper noun is the name of people, places or organisations.
We use capital letters for :
for proper nouns:
Tony Blair, Birmingham, China, the United Nations, …
Christmas; Deepawali; Easter; Ramadan; Thanksgiving, …
for a title:
I was talking to Doctor Wilson recently.
Everything depends on President Obama.
Call the Professor Johnson.
for the names of books, films, plays and paintings:
=> we use capital letters for the nouns, adjectives and verbs in the name:
I have been reading ‘The Old Man and The Sea’.
You can see the Mona Lisa in the Louvre.
for an artistic creation:
Recently a Van Gogh was sold for five millions dollars.
We were listening to Mozart.
I’m reading an Agatha Christie.