Get your paperwork (like visa and application) done before you do anything. If your passport has expired or you have yet to obtain one, get it without wasting any more time. Apply for a tourist visa. Keep in mind that Australia requires travelers from other countries (excluding New Zealand) to produce a visa to enter the country. Tourist visas are easy to get as all you have to show enough money and the right reason (which is tourism of course) to the visa officers.
Make a budget that is viable and does not hurt your financial capability. Making a budget will take into account several things and missing main expenses might disrupt your entire journey and plan. Write down all the main expenses like traveling (that include visa and paperwork fees, air tickets, rail, car, inter-country flights and train travel), accommodation, food, shopping and miscellaneous expenses like entertainment. The budget should be prepared well before your actual date of departure as you may need to make necessary changes according to your financial strength. It is up to you to decide how much you will want to spend on each expense. Perhaps, traveling is more important you than accommodation and your plan is to the travel through cities, villages and towns. In that case, you will definitely want to spend more on traveling than other things like accommodation and entertainment.
Decide the month or date of your journey. This is important for your trip as you should know that seasons in Australia are reverse compared with the rest of the world. For instance, winter falls during June and July. During December, when people in your country face cold winds and snowy weather, people in Australia have to face scorching heat that may see temperatures touch as high as 40 to 50 degrees C.
Visit official tourism and plan a trip. Official tourism site of Australia will save you a lot inconvenience, but you may not be able to shape your budget as the expenses may exceed your expectations.