The first week
If you are off work for less than seven days, you do not need a sick note from your doctor. You should complete the appropriate self certificate form available from your employer or on the HMRC website https://www.gov.uk/government/publications/statutory-sick-pay-employees-statement-of-sickness-sc2
After the first week
If you have been off work for more than a week you will need a sick note from your doctor. You can do this by contacting reception for an appointment.
Statement of Fitness for Work - ’Fit Note'
The 'fit note' was introduced on 6 April 2010. With your employer's support, the note will help you return to work sooner by providing more information about the effects of your illness or injury.
For more information see the DirectGov website (where this information was sourced).