All travellers to Australia, except New Zealanders, require some kind of a Visa to visit Australia.
If you have a valid passport from one of the 32 eligible countries, are outside of Australia, of good health and no criminal convictions you can apply online from this site for your Visa (also known as an eVisa/s).
It is not necessary to have a trip booked before being issued with your Visa. In fact, the Australian Government recommends your Visa should be arranged before booking any part of your trip to ensure approval has been granted to visit Australia. More Details.