You are not required to provide a self certificate for the first 7 days of any absence due to illness.
Your employer may ask you to complete a self certificate form SC2; they can supply this to you or you can download from http://www.hmrc.gov.uk/forms/sc2.pdf
If your illness results in an absence of 8 or more days, you should arrange an appointment with a doctor and request a sickness certificate (MEd3). These are free of charge. Please note the practice nurses cannot issue sick certificates.