According to most style guides, nouns, pronouns, verbs, adjectives, and adverbs are capitalized in titles of books, articles, and songs. You’d also capitalize the first word and (according to most guides) the last word of a title, regardless of what part of speech they are.
A few parts of speech tend to be lowercase. For instance, articles (the, an, and a) are lowercase.
Some conjunctions (e.g., but, yet) and prepositions (e.g., over, through) are capitalized, and sometimes some are lowercased—it depends on the style guide you follow. For instance, in AP style title case, prepositions of four letters or more are capitalized. But in Chicago-style title case, all prepositions are lowercase, no matter their length. When in doubt, look up the rules of the style guide you’re choosing to follow to know exactly how to style a title.
Articles:
Let’s start with articles. All major style guides are aligned on articles, which agree that articles are lowercase in titles unless they’re one of the first and last words:
Incorrect: Ernest Hemingway wrote For Whom The Bell Tolls.
Correct: Ernest Hemingway wrote For Whom the Bell Tolls.
Incorrect: Girl on A Train is a thriller by A. J. Waines.
Correct: Girl on a Train is a thriller by A. J. Waines.
In each of these titles, the and a are not capitalized because they are articles.
In this next example, the first article, a, is capitalized because it’s the first word, but the is lowercase because it is an article that is not a first or last word:
Incorrect: Jennifer Egan wrote A Visit from The Goon Squad.
Correct: Jennifer Egan wrote A Visit from the Goon Squad.
Not Capitalized in Titles:
The word the is only capitalized if it is the first word in the title.