You have to make deductions on amounts you pay if you are an employer, a trustee or a payer. After you have made the deductions, you have to remit these deductions, plus your share, to us. You then report the employee's income and deductions on the appropriate information return.
See Employer responsibilities - The payroll steps for a definition of the above terms, and to find out what to do for each step.
If you are operating in Quebec, you also have provincial requirements. See Revenu Québec.