Just found out I had to pay tax after renting out a property for nearly a year. Where do I go about paying the tax?

http://www.hmrc.gov.uk/

Everything you need will be there.

Remember that there will be a range of "allowable expenses" that can be used to offset some of the income and so reduce the tax bill. I am still trying to find out exactly what are the allowable expenses. I think things like annual Gas/ Electricity safety certificates, essential repairs are allowed. I am still not sure about the status of property insurance. In my case the rental is handled by an agent and their fees can be used to offset some income for tax purposes

Snod

If it's a low amount of tax, HMRC will give you the option of adding it to your PAYE code (assuming you're on PAYE) so they can recoup it that way. Otherwise you'll have to file a Self Assessment return.

Allowable expenses to reduce your liability include any mortgage interest you've paid, landlord/house insurance, letting agent fees, gas/electric certs and the cost of advertising your property. There's a 'home office' allowance of £208 and you can also claim against any trips you've had to make to your property - 45p per mile assuming you used a car.