These links are a starting point when considering freelancing. Consult an accountant or your country's tax authorities for advice.
The Australian Taxation Office (ATO) website provides a wealth of information about Starting your own business. Sole Trader is the simplest option to start. There is also an Australian Government Business website which provides information about Payments and invoicing, Superannuation, and Taxation.
This salary calculator can help you estimate how much tax, national insurance and student loan you may need to pay.