Uncountable nouns require no article:

water, milk, rice, tea

Countable nouns (and adjectives modifying nouns) that begin with consonants require "a":
a pen, a book, a car, a cat

Countable nouns (and adjectives modifying nouns) that begin with a vowel require "an":
an apple, an egg, an umbrella, an octopus

an = a

"The" is used after a noun is introduced with "a" or "an".
a cat.
The cat is small.

"The" is also used when describing something that both the speaker and the listener know about.
I went to the park yesterday (the park both people usually go to).

"The" is also used when there can only be one thing.
He is the best.
She is the tallest.

Don't use "the" before:
1. the names of cities, towns, or states: Kyoto, Chicago, Houston
2. the names of streets: Madison Ave., King St.
3. the names of lakes and bays: Lake Michigan, Lake Ontario
4. the names of mountains: Mount Kilimanjaro, Mount Fuji
5. the names of continents: Antarctica, South America
6. the names of islands: Kodiak Island, Oahu
7. the names of countries: Spain, Canada, Thailand
(except for the Netherlands, the Bahamas, the United States, etc.)

Do use "the" before:
1. names of rivers, oceans and seas: the Amazon, the Atlantic, the Red Sea
2. specific points on the Earth: the Prime Meridian, the South Pole, the Equator
3. geographical areas: the West Bank, the East Indies
4. deserts, forests, gulfs, and peninsulas: the Gobi Desert, the Gulf of Mexico, the Black Forest, the Texas Panhandle