Your employer (if you have one) will tell the tax office that you work for them. You will receive an income tax form from the Inland Revenue which you must fill in and return within thirty days (hence the name 'return'). See http://www.hmrc.gov.uk/sa/index.htm
If your only income is from a job and you are taxed under PAYE by your employer, you may only receive a form once in every three years. The tax office uses it to check that you are still employed, and whether circumstances have changed. The employer does the regular work of giving the inspector of taxes information and paying the collector of taxes (by deducting income tax from your pay before paying it to you). You only have to deal with changes and additional income or allowances.
Remember that it is an offence not to fill in and return your income tax return on time. The onus is on the taxpayer to disclose income; therefore if you have a new source of income which you have not notified to the inspector, you should ask for a tax return or explain the circumstances in a letter.