You must contact HMRC if you have taxable profits from renting out your property. If you have not told HMRC about your property rental you must do so by 5 October following the tax year you had rental profits. If you do not you could be charged a penalty.
Rental profits are usually reported to HMRC on a Self Assessment Tax Return. If you are also employed and your rental profits are low enough you can request that HMRC adjust your PAYE code.
There are different ways to register with HMRC depending on whether you are self employed or a sole trader, not self employed or a member of a Partnership.
There are many different rules if you are renting a room in your house, letting a property as a furnished holiday letting, renting out foreign property or letting a property in the UK whilst you are abroad.