The forms SC1/SC2 (the “self certificate claim form for sickness benefit”) can be downloaded from Statutory Sick Pay guidance and forms for employees and employers - GOV.UK (www.gov.uk). Both these certificates will cover you for 7 days. If you are off work for more than 7 days you will need to get a medical certificate (sick note) from the doctor.
Please note that there is a charge for a private sick note.