Online encyclopaedias are great for short, concise and reliable information. It may be best to access these articles from the Online encyclopaedias page through the Hale portal. Don't forget Britannica has ready made citations for you to use.
It is important to provide evidence of using a variety of reliable resources. Use the online Reference Generator available through the Portal to create your citations. Make sure you alphabetically sort them afterwards.
A sample bibliography for 3 resources suggested on these pages would look like:
Battle of Thermopylae 2019. Britannica School. Retrieved 17 May 2019, from https://school.eb.com.au/levels/high/article/Battle-of-Thermopylae/473472
MacDonogh, G 2010, The great battles, Quercus, London.
‘Sparta’ 2019, in Ancient History Encyclopedia, accessed 17 May 2019, <https://www.ancient.eu/sparta/>.