As an International student filing for the first time, I filed using turbo tax and later found out that I did not file as a non-resident because I did no use form 1040nr what is the next step to take please?

You need to file amended tax return. For amending your tax return, you must file IRS Form 1040-X with properly done nonresident tax return Form 1040NR or 1040NR-EZ and Form 8843. Column A is what was on the original return, Column C is what should have been there (that is corrected amounts) and Column B is the difference. Complete these columns and then refigure your tax.
Read: Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents (http://taxipay.blogspot.com/2008/03/us-tax-filing-requirements-for-non.html)