A capital letter is used as follows: 

To begin the first word of every line of poetry.

To begin the first word of every sentence.

To begin a word indicating a personification.

To begin the names of days and months.

To begin a proper Noun.

To begin all Nouns and pronouns referring to God.

To begin all ranks, titles, degrees, designations, languages, subjects, religions, races, castes, parties, institutions, and associations.

For the Pronoun ‘I’ and Interjection ‘O’.

Example:

Man is mortal.
Youth is hot and bold.
Make hay while the sun shines.
Money makes the mare go.
They never die who die for a noble cause.