You must apply for and receive an Employment Authorization Document (EAD/I-797D) from USCIS with the assistance of the DSO in OIS before you can begin work.
The application must be properly filed with the USCIS before the end of your 60-day grace period following your program of study. Download Form I-765 from the USCIS website. Click on the Immigration Forms tab and scroll down to Form I-765.
You must begin the application process sufficiently in advance to allow the DSO to update the SEVIS record with the recommendation, and to ensure that the I-765 is accepted by USCIS prior to your program completion date. USCIS will accept applications up to 90 days before the end of your program.