It is important to work on your writing, but you should also know how to reference quotations and authorities. There are basically two systems. The first is the old-fashioned but perfectly useful footnote system, by which you direct readers to important material, written out in full at the bottom of the page or at the end of an article, chapter or book. The other way is the Harvard system, in which you use a shorthand in the text itself (such as Harvey, 2005), with the full information at the end of the document, like this:
Harvey, D. (2005) A Brief History of Neoliberalism. Oxford: Oxford University Press.
Katz, C. (2001) ‘Vagabond capitalism and the necessity of social reproduction’, Antipode 33(4), 709-728.
Smith, N. (2003) American Empire: Roosevelt’s Geographer and the Prelude to Globalization. Berkeley: University of California Press.
As above, these bibliographic references should always be in alphabetical order, as a courtesy to the reader. Multiple references in brackets should also be in alphabetical order (such as Harvey, 2005; Katz, 2001; Smith, 2008).
Both footnotes and the Harvard system are useful – the latter more for natural science and social science publications, the former rather more so for arts and humanities. In general, use the Harvard system; but as long as you are consistent, that’s fine. It is also possible to combine them, but then footnotes should be used very sparingly.