Firstly you need to check that you qualify for statutory maternity pay. You need to be an employee and have been continuously employed for at least 26 weeks by the end of the Qualifying Week, which is the 15th week before the expected week of childbirth (EWC). Your normal weekly earnings should be not less than the Lower Earnings Limit for National Insurance purposes which changes yearly like the rate of maternity pay.
In addition you must still be pregnant 11 weeks before the start of the EWC (or have already given birth). You must also supply a certificate from your doctor or midwife (normally the MAT B1) and give your employer at least 28 days notice (or, if that is not reasonably practicable, as much notice as is reasonably practicable) of the date you intend SMP to start. You must then stop work.
If you do not qualify for maternity pay then you may qualify for maternity allowance instead. Assuming you do qualify then the rate as at April 2015 is first six weeks at the highest of 90% of normal pay or £139.58 per week. The following 33 weeks if you take your full maternity paid leave is then at the rate of £139.58. Parents whose baby was born on or after 5th April 2015 may take shared parental leave so if you cut short your maternity leave this will no longer be payable. Instead you and/or your partner may qualify for a different payment.
Some employers offer an enhanced maternity pay scheme which is more generous.