Capital letters are used:
1.    At the beginning of a sentence –

Alice is a good girl.
He is a clever boy.
She is very efficient.
They are coming.

2.    At the beginning of each line of poetry –
I am a teapot
Fat and Stout.
This is my handle,
And this is pout.

3.    For names of people, places, rivers, mountains etc.
Alice, John, Tigris, Danube, Mt. Everest

4.    For names of nations and adjectives of nationality –

India, Japan, Germany, Indian, German, Persian

5.    For names of months, days, festivals and historical eras –

Saturday, October, Christmas, Easter, the Middle Ages

6.    For names of books, plays, famous paintings etc.

the Last Supper, the Mahabharata, the Bible

7.    For all adjectives derived from proper nouns –
Persian, Hindu, Himalayan, Christian, Biblical

8.    To write the pronoun I and the interjection O -