If you have Self Assessment income tax to pay based on your personal Self Assessment tax return this will normally be due by 31 January following the end of the tax year concerned.
You may also need to make payments on account for the following year and these will be in two installments, by 31 January and by 31 July.
There are various methods which you can use to make your tax payments and the link below will take you to the HMRC web page that lists and describes these methods: http://www.hmrc.gov.uk/payinghmrc/selfassessment.htm
There are also links from this page to the HMRC online payment facility if you wish to pay with a debit card.
Your payment reference is the 10 digit UTR number shown on the front of your tax return (followed by a letter K).