Harvard referencing is the most common referencing style in academia and is the style suggested in the EPQ specification.
There are free online reference generators, for example: http://www.citethisforme.com/uk/referencing-generator/harvard
However if you want to be sure and do it yourself the Haravard referencing style is as follow.
Books
Last name, first initial. (Year). Title. Edition (if not the first edition of the book). City of publication: Publisher.
Example
Hawking, S. (2005). A Briefer History of Time. 2nd ed. London: Transworld Publishers.
Articles
Last name, First initial. (Year). Article Title. Journal name, Volume (Issue), Page/s.
Example
Jones, D. (2011). String Theory as a Mode for Time Travel. Reviews of Modern Physics, Volume 3 (1), pp. 12-14
Websites
Author/Source if no specific author (Year). Title of web document/page. [online]. (Last updated: if this information is available). Available at: URL [Accessed date: Day/Month/Year].
Example
Nerd Herd Corner (2019). How to build a time machine. [online]. Available at: https://nerdherdcorner.com/2019/01/10/how-to-build-a-time-machine/ [Accessed 7 Feb. 2019].