There are a number of online tools that can help you create a legal citation in Bluebook format. None of them are perfect, so double-check your results against the actual Bluebook manual to be completely sure they're correct.
1) Use the "Copy with Reference" tool in the Westlaw database. There is another tab on this screen (above) where you can find instructions for setting up the "Standard" citation preference in Westlaw, which is the same thing as Bluebook.
2) Visit the Bluebook's official website for some helpful tips.
3) This selection of legal citation generators was compiled by the Gallagher Law Library at the University of Washington School of Law. Their amazing research guide also contains this collection of instructional videos about using Bluebook format.
4) The free citing tools on the "Cite This" website include Bluebook format, but you need to know what each of the elements of the citation are in order to use it.