Capital letters for proper nouns
Use capital letters for proper nouns, but not for common nouns.

Capital Letters for Proper Nouns

Use a capital letter for the names of people, places, planets, days of the week, titles of rank or relationship (when joined to personís name), months, holidays, departments, clubs, companies, institutions, bridges, buildings, monuments, parks, ships, hotels, streets, historical events and documents.  (These are known as proper nouns.)  Do not use a capital letter for a common noun, unless it starts a sentence.

Examples (proper nouns in bold):

The next lake the party visited was Lake Michigan.
(The word lake is a common noun; it is the word for an in-land water feature.
Lake Michigan is a proper noun; it is the name of the lake.)

According to Lord Davies, The Church in London is not actually a church; it is†a public house.
(The Church - proper noun; church - common noun)
It was a rewarding day, and I intend to visit here again on Armistice Day next year.
(day - common noun; Armistice Day - proper noun)
Could you ask Sergeant Allan or the other sergeant to arrange the patrol
on Friday morning?
(Sergeant Allan - proper noun; sergeant - common noun)

Big Fish should be big fish
(magazine article)

Toilet should be toilet
(instructions on packaging for a toilet seat)

A common noun is the word for something (like tower). A proper noun is the name of something (like The Eiffel Tower).

See the lesson: Different Types of Nouns

Do not be tempted to give a word a capital letter just because it is an important word in your sentence.

Place your order using the form in our latest Brochure.
We value our Clients' opinions.

I'm having the best day of my life, and I owe it all to not going to Church! (Homer Simpson)

Lisa, Vampires are make-believe, like elves, gremlins and Eskimos. (Homer Simpson quote)
(Vampires is wrong. Eskimos is correct.) 

We live in an age when Pizza gets to your home before the Police.

In business writing particularly, it is considered good practice (for politeness) to capitalize some common nouns, such as Company and Director – you have a choice.

The manager of your company confirmed the booking in writing on 15 August.
The Manager of your Company confirmed the booking in writingon 15 August.

In the main, this pertains to job titles and names of departments. Other words which may be important enough in the context of your writing to warrant capital letters are:

Director, President, Office Manager, Commanding Officer, Division, Claims Department, Court, Regiment, Unit

The director will cast the final vote.
The Director will cast the final vote.
The Director will cast the final Vote.

See also:

What are nouns?
The different types of nouns
Capital letters in advertisements
Capital letters and the points of the compass
Capital letters with the four seasons
Capital letters with moons, stars and planets
Capital letters start sentences
What is title case?
Test on using capital letters