The colon can be used to punctuate sentences in different ways.
It can be used to introduce a list.
Please bring these items with you: a packed lunch, a drink, a sunhat and a raincoat.
Colons should only be used to introduce lists if the section of the sentence before the list would make sense on its own. In the next example, 'Please bring' does not make sense on its own, so a colon is not needed.
Please bring a packed lunch, a drink, a sunhat and a raincoat with you.
A colon can also be used to introduce a quotation.
My chemistry teacher had a favourite saying: 'Do as I say, not as I do.'
Again, the colon should only be used if the part of the sentence before the quotation makes sense on its own. In the following sentence, 'My chemistry teacher used to say' doesn't make sense on its own, so a colon is not needed.
My chemistry teacher used to say, 'Do as I say, not as I do.'