Usually taught in the first year of education, a capital letter is used at the start of a sentence, and for names and titles/headings. This includes the names of people, organizations, places, days and months, and (often forgotten) the name of a language, brand or ethnicity. There are a few other examples, which a quick Google search will explain. It is considered acceptable to use block capitals instead of italics to emphasize a specific word or point. It is retarded to use capitals for every word, or just your favorite words, or sporadically throughout a text.
1. In other news, the Prime Minister of Sweden visited Washington today, and my tiny little nipples went to France.
2. My assignment is in on Tuesday, Peter. I am going to Wikipedia it so that I progress no further with my education because I prefer to go on Facebook, buy myself presents on eBay and watch Sons of Anarchy online :).
(note for titles: 'joining' words such as 'and', 'of' and 'the' don't need capital letters)
Incorrect use of capitalization:
1. Lame Facebook status: 'Just Remember You Are Always A Shining Star'
2. Person 1: 'JOHN ARE YOU GOING TO THE CINEMA TONIGHT?'
Person 2: 'Not if you ask me in caps lock... not cool man...'
3. 'Omg why aren't we Allowed to wear fake tan to school anymore? I like being orange!!!' (allowed did not require a capital letter for ANY REASON)