All works cited or quoted must be properly referenced in the text, and listed in a bibliography at the end of the essay - remember, plagiarism is a hanging offence! In any case, routine inclusion of references and preparation of a bibliography are useful habits to develop in your approach to writing.
Each reference will usually be in one of the following forms:
For books:
Cunliffe, B.W., 1984, Excavations at Verylargebury, Megatome Press, Oxbridge
Blackadder, E., 1588, The Cunning Plans of my Servant, Baldrick, Nonsuch Press, London
(NB there are variations on this.
For articles in journals:
Khan, K., 1277, ‘Stately pleasure-domes I have known’, The Xanadu Archaeological Journal, Vol. CXIII, pp. 1-299.
For a paper in an edited volume:
Stalin, J., 1992, ‘They wouldn’t have got away with it in my day’, in: Engels, F., Lenin, V.I., Marx, K. and Trotsky, L. (eds.), It seemed a good Idea at the Time, Winter Palace Press, St. Petersburg, pp. 123-456.
Within the text of your essay, the ‘Harvard system’ is now more or less standard as a way of referring to published information on which the statements in your essay are based. (Footnotes should usually be avoided, even though these are now pretty easy to do with advanced Word Processors).
The standard form of such a reference, for material referred to or (sparingly!) quoted verbatim is:
‘The hillfort proved to contain thousands of pits (Cunliffe 1984, p.123).’
There are variations to note.
‘Cunliffe reported that the hillfort contained thousands of pits (1984, p.123)’ is another way of expressing the point above, and makes for a little variety.
Two authors/editors would be referred to as ‘...(Baldrick and Blackadder 1588, p.567).’
If there are more than two authors/editors the reference is usually presented as ‘...(Engels et al. 1992, p.789).’ where ‘et al.’ is abbreviated Latin et alii, et alia, etc. meaning ‘and others’. (See the full bibliographical reference to this fictional volume above.)
Tip: Have a look at how authors use the system in published works. You will find there are minor variations in usage, but you will soon get the hang of it.