Did you inhertiate the house with the fixtures? If so (to the best of my recollection) there would have been an estimate put on the fixtures and fitting that were in the house....so you use this figure (plus anything else you many have added into the property....and minus anything you took out of the property) and then you write it off at a rate of 12.5% pa.
If it's you first year, it might be worth getting an accountant to do the returns for you so you will know what to do in future and be certain you are allowing for all appropriate expense, capital allowance, PRTB is up to date, e.tc. Generally fairly straight forward once you have an idea of what to do