You can only begin employment if you have received authorization (see 3rd page of your I-20) from Student Services .
CPT is approved per work experience . Once you receive approval, you will receive an I-20 with the work authorization details including the name of the company and approved dates. You will need to show your employer the third page of the I-20 with CPT authorization to begin working.
Working without authorization or before or after the approval dates is a violation of status and will result in the loss of your F-1 status.