Use capital letters for proper nouns. Avoid them for common nouns.
Proper nouns are the names of things. So use capitals for the names of people, places, months, days of the week, companies and so on. Don’t use capitals for common nouns.
People run into difficulty with capitals because there’s a temptation to use them for important words. In business writing people often use capitals as a way of avoiding offending someone or something by implying he or it isn’t important.
Another difficulty is with titles. Newspapers typically use a capital letter when the title comes directly before a person’s name but not otherwise.
New Zealand Prime Minister John Key is correct, but it would be the prime minister’s desk.
In his book Newsman’s English British newspaper editor Harold Evans says;
“Avoid using them unnecessarily. The Parks Committee, but subsequently the committee. The South West Regional Hospital Board, but then the hospital board.”
One piece of advice I had early in my career as a journalist is: “If in doubt use lower case unless it looks wrong”.
Lastly, do not use capital letters for emphasis and avoid writing words in all capitals.