First, check with your lecturer to see if there is a preferred referencing style for your assignment. If they don't have a preference, you can then choose one of the referencing styles used at the University of Sydney, see Referencing and Citation Styles.
This guide will show you examples of how to reference using each style. It also refers you to more in-depth and complete guides that you can use for less common reference types and situations.
Once you are familiar with the referencing style you plan to use you may also like to join an Endnote class. Endnote is a program that allows you to collect, organise your references and insert them into your document in your chosen referencing style.