Here are the basic rules for what words you should capitalize
The first word of every sentence
The first-person singular pronoun, I.
The first, last, and important words in a title. (The concept "important words" usually does not include articles, short prepositions (which means you might want to capitalize "towards" or "between," say), the "to" of an infinitive, and coordinating conjunctions. This is not true in APA Reference lists (where we capitalize only the first word), nor is it necessarily true for titles in other languages. Also, on book jackets, aesthetic considerations will sometimes override the rules.)
Specific persons and things: George W. Bush, the White House, General Motors Corporation.
Specific geographical locations: Hartford, Connecticut, Africa, Forest Park Zoo, Lake Erie, the Northeast, the Southend. However, we do not capitalize compass directions or locations that aren't being used as names: the north side of the city; we're leaving the Northwest and heading south this winter.
When we combine proper nouns, we capitalize attributive words when they precede place-names, as in Lakes Erie and Ontario, but the opposite happens when the order is reversed: the Appalachian and Adirondack mountains. When a term is used descriptively, as opposed to being an actual part of a proper noun, do not capitalize it, as in "The California deserts do not get as hot as the Sahara Desert."
Names of celestial bodies: Mars, Saturn, the Milky Way. Do not, howver, capitalize earth, moon, sun, except when those names appear in a context in which other (capitalized) celestial bodies are mentioned. "I like it here on earth," but "It is further from Earth to Mars than it is from Mercury to the Sun.
Names of newspapers and journals. Do not, however, capitalize the word the, even when it is part of the newspaper's title: the Hartford Courant.
Days of the week, months, holidays. Do not, however, capitalize the names of seasons (spring, summer, fall, autumn, winter). "Next winter, we're traveling south; by spring, we'll be back up north."
Historical events: World War I, the Renaissance, the Crusades.
Races, nationalities, languages: Swedes, Swedish, African American, Jewish, French, Native American. (Most writers do not capitalize whites, blacks.)
Names of religions and religious terms: God, Christ, Allah, Buddha, Christianity, Christians, Judaism, Jews, Islam, Muslims.
Names of courses: Economics, Biology 101. (However, we would write: "I'm taking courses in biology and earth science this summer.")
Brand names: Tide, Maytag, Chevrolet.
Names of relationships only when they are a part of or a substitute for a person's name. (Often this means that when there is a modifier, such as a possessive pronoun, in front of such a word, we do not capitalize it.)
Let's go visit Grandmother today. Let's go visit my grandmother today.
I remember Uncle Arthur. I remember my Uncle Arthur. My uncle is unforgettable.
This also means that we don't normally capitalize the name of a "vocative" or term of endearment:
Can you get the paper for me, hon?
Drop the gun, sweetie. I didn't mean it.