"Not" in British English. "I have a car" negates to:
formally: "I do not have a car"
informally: "I don't have a car" / "I haven't got a car"
Optional further detail:
Out of interest, the negation of "I have a" to "I have not a" was historically correct as late as the 1800s (see Google Books http://www.google.co.uk/search?hl=en&;cli… ) but it has largely dropped out of the language. However, there are a few contexts where it's still used - for instance, "You haven't a chance of winning", "I haven't any money", and so on - but you needn't worry about those because "I don't have ..." is reliably correct.