You can only begin employment if you have received authorization (see 3rd page of your I-20) from Student Services after submitting the following documentation (see application instructions):
CPT is approved per work experience. Even if the employment is only for one day, you will need to apply for CPT authorization and only work the days indicated on the I-20. Working before or after the approval dates is a violation of status and will result in the loss of your F-1 status. You will need to show your employer the third page of the I-20 with CPT authorization to begin working.