The punctuation mark colon (:) is almost always used after a complete sentence. Its function is to indicate that what follows is an explanation or elaboration of what precedes.
We decided not to go on holiday: we had too little money.
Mother may have to go into hospital: she has got kidney trouble.
A colon can introduce a list.
We need three kinds of support: economic, moral and political.
A colon is never preceded by a white space, and it is never followed by a dash or a hyphen.
In British English, it is unusual for a capital letter to follow a colon (except at the beginning of a quotation). However, this can happen if a colon is followed by several complete sentences.
In American English, colons are more often followed by capital letters.