Statutory Sick Pay (SSP) applies after three days of illness or incapacity for work. These three days may, but do not usually, include non-working days (i.e. weekends or days not normally worked). All employees are entitled to 28 weeks SSP.
Working days lost prior to SSP commencing are paid at the employer's discretion. This will usually mean payment at the normal rate of pay but this will depend upon the circumstance and any terms of employment already agreed. Thereafter SSP can be paid instead of or as a part of the normal rate of pay.
Your employee should provide you with a doctor's note or letter confirming days absent through sickness. You are entitled to Small Employer's Relief - i.e. to claim back a proportion of SSP paid. Enable Payroll will help with this.