I want to publish an autobiography book about my bad childhood, how to do so.?

Not sure about in Australia but in the USA once you have finished the book and you have proof read it a million times the next step is to either find a publisher or a literary agent, since most publishers want to speak to an agent not the author. It WILL take many rejection letters before you find one who is willing to even give you a shot but if you keep trying and its a good story you will find that perfect match. Unless you are self-publishing it doesn't really cost you money to get your book out there. Good Luck!