A capital letter signals to the reader that a word needs special attention. A good rule of thumb is that you should not capitalize the first letter of a word unless you know of a rule that says that you should. The following rules cover most of these circumstances.

Capitalize the first word in every sentence.
Capitalize the pronoun I and the poetic interjection O.
Capitalize proper nouns and adjectives (examples: Pensacola State College, Montana, Chinese, Elizabethan)
Capitalize the names of individual persons, including titles used before their names (example: Chancellor Robert Talmadge, the Reverend Bob Bessemer).
Do not capitalize compass directions, but capitalize the words when they refer to regions of a country or the world (examples: north, southeast, west, but the South, the Far East, the West Coast).
Capitalize all important words in the specific names of organizations; institutions; government bodies; business firms; brand names of business products; buildings; ships; planes; trains; special events; historical events and eras; items on the calendar (but not seasons of the year); races; religions; tribes; and nationalities.
Do not capitalize the names of school subjects, except for proper nouns and adjectives (examples: geometry, French, American history, biology). Course names followed by a number are usually capitalized (examples: algebra, but Algebra II).
Capitalize the first word, last word, and all other important words in titles of books, magazines, newspapers, articles, laws, movies, historical documents, TV programs, and works of art. “Important words” include all words except the articles (a, an, the), coordinating conjunctions (and, but, or, nor, for, so, yet), and short prepositions (fewer than five letters long). Some examples are the following: To Kill a Mockingbird, the United States of America, The Trouble with Tribbles, The Bill of Rights, Pensacola News Journal.
Do not capitalize grade-level classifications unless they are used in the name of a specific organization (examples: sophomore, senior, Junior-Senior Prom, Sophomore Class Board)
Capitalize words referring to the monotheistic Deity (example: God, His grace, gods of the Greeks and Romans).