You use the word 'get' when you are looking to receive something. An example would be "Let's get ice cream". You use the word 'have' when you already obtained something. An example would be "I have the money".

I would offer a slightly different description from what adrian wrote. "Get" is somewhat informal and is used more in conversation (or email) than in formal writing such as a business document. I think you would see it in newspaper articles and opinion pieces. I haven't thought about this deeply but I'm guessing that there is always a more formal way to write any sentence that uses "get". This dictionary entry provides a pretty good list of the uses of "get". http://dictionary.reference.com/browse/get NOTE: Keep in mind that even the uses of "get" that the dictionary entry does not label as "informal" are still relatively informal. Still, "get" is widely used in English conversation so it's really worth getting to know how to use the word.

Do be aware that "get" is very informal and pretty much only used in conversation...Definitely not for any essays..articles..any formal writing. :)