The Department of Home Affairs runs a nifty web page with all visa options. In a first step you name your main reason for travel. Then you tell which country issued your passport. Then you receive options.
If you plan on staying less than three months in the Commonwealth of Australia, chances I you can apply for an eVisitor or for an Electronic Travel Authority (ETA).
If you are from Europe or Taiwan, you will apply for eVisitor. If not, apply for an ETA.
Make sure to apply via the official website of Home Affairs. Don’t spend money on third party websites. Even if they are higher up on Google.