Please follow this link to the U.S. Office of Personnel Management website for your retirement needs: 

www.opm.gov/retire/retire_employees.asp

Required Forms for Retirement

SF2801 Application for immediate retirement (CSRS)

SF3107 Application for immediate retirement (FERS)

SF2818 Continuation of Life Insurance Coverage as an Annuitant or Compensationer

W-4P  Withholding Certificate for Pension or Annuity Payments