For the first seven days of sickness, you need to fill an SC2 form which is found here.

After seven days, you will need an NHS certificate.

If you have been in hospital (e.g. for an operation), it should be issued by the hospital for the expected length of recovery. See DWP guidelines here.

If you are being treated by a GP then they will issue the sick note.

For information on claiming benefits due to illness you can look here.

Department of Work an Pensions FAQs