There is no stated minimum amount of time you have to be out of North America (not just the USA) before you can return for another visit on the Visa Waiver Program. You could leave one day and return the next.
HOWEVER, Immigration watches for people who try to illegally live (or worse, work) in the USA on the VWP. If you return quickly after a long visit you can expect to be closely questioned and may be denied entry if you don't have a good explanation of the reason for your return. "Tourism" may not be enough unless you have significant documentation your visit really is for tourism.
Citizens of about 35 countries can travel to the USA for up to 90 days at a time as a tourist or for some business purposes by registering for ESTA > before < their trip begins. The cost is US$14, which is similar to the fee for the Australian ETA program.
Registering is accomplished online, you don't need the details of your trip, and the confirmation is valid for multiple trips to the USA over two (2) years so don't wait until the last minute.
Print the confirmation - or at least record the number - and keep the information with your passport. Having a return or onward ticket leaving North America is a requirement, so have a copy of your airline itinerary at passport control.
Check with your health insurance to find out if it covers you in the USA. If yes then take proof of coverage with you. If no or not sure then get trip medical insurance, which is cheap and sold by airlines & travel agents. The USA has wonderful medical care but it isn't free or even cheap. Please do not skimp on this as an otherwise-silly accident could turn into a crisis if you don't have medical insurance. This is a US government site, but the information is valid for anyone traveling away from their home country:
If you are planning a long visit then DON'T schedule your departure day for the 90th day (any part of a day in the USA counts as a whole day, so if you arrive at 23:59h on the 1st and leave at 00:01h on the 10th it is 10 days in the USA). Schedule your departure not later than the 88th day as there are no (zero) provisions in the Visa Waiver Program for someone to voluntarily extend their stay. If you miss your flight due to something Immigration might consider avoidable (ex. flat tire on the way to the airport) they may ban you from using the VWP in the future.