If you have an ordinary, paying job, it will most likely be the responsibility of your employer to make sure they are deducting the correct amount of tax from your pay and sending it to the Australian Tax Office.
If you are uncertain whether this is happening correctly, you can contact the Australian Taxation Office (ATO) Call Centre for information between 8am and 6pm weekdays on 13 28 61 from anywhere in Australia.
The amount of tax you pay depends on how much you earn in a year. If you want to figure it out, the current rates of tax are set out on this ATO web page:
http://www.ato.gov.au/individuals/content.asp?doc=/content/12333.htm
Make sure you ALWAYS keep your pay slips so there is a record of what you have been paid, the hours you have worked, and the tax which has been deducted.