Your only automatic entitlement to receive sick pay relates to 'Statutory Sick Pay'. This is currently �72.55 per week. It's payable from the 4th day of your sickness and lasts for up to 28 weeks. You can initially 'self-certify' your sickness, using this form:
http://www.hmrc.gov.uk/forms/sc2.pdf
You should do so within 7 days of the sickness first occurring.
For lengthy periods of sickness, an employer can insist that the employee provides further evidence, either from their GP or from a hospital.
http://www.directgov.gov.uk/en/MoneyTaxAndBene fits/BenefitsTaxCreditsAndOtherSupport/Illorin jured/DG_10018786
Some employers have their own sick pay schemes, which must pay at least as much as SSP. If your employer has such a scheme, the actual amount will be determined by your the conditions in your contract of employment (or by any collective agreement between your employer and your trade union). Some schemes are very generous (e.g. teachers usually get full pay for at least 6 months and at least half-pay for a further 6 months). Others might, say, only provide half-pay for 3 months and SSP thereafter. There are no fixed rules and, as indicated, many employers don't have such schemes and only pay SSP.
Chris