If you were legally adopted under full Hague Convention arrangements, you can apply for citizenship by descent using form 1272 'Application for Australian citizenship for children adopted under full Hague Convention arrangements'.
http://www.immi.gov.au/forms/Documents/1272.pdf Form 1272
Otherwise, you can only apply for citizenship by descent if your parent was an Australian citizen at the time you were born AND was your parent at the time of your birth so if you were not legally adopted, you are not eligible.
If you are still a full time student, your step-father can sponsor you for a Child visa which is a permanent visa. After living in Australia for 4 years, you would then be eligible to apply for citizenship.
http://www.immi.gov.au/Visas/Pages/101.aspx