(2-part post):
First, check if there's actually a will.
If there's a will, the executor of that will (which could be you but might, for example, be the solicitor of the deceased person) needs to apply for probate.
If there's no will, the house will become yours if you're the sole
direct descendant of the deceased person. However, if you're more distantly related, you should keep in mind the fact that (under some circumstances) anyone descended from a grandparent of the deceased relative might be entitled to a share of the estate (which includes the house). So there might be someone who you don't think of as a 'descendant' of the deceased person, but who's still entitled to receive something. (If in doubt, consult a solicitor).
Even if there's no other person entitled to receive a share of the estate, the house isn't legally yours yet. If there's no will, you need to apply for letters of administration.
Either way, you need to contact the relevant probate registry as soon as possible:
http://www.hmcourts-service.gov.uk/infoabout/c ivil/probate/registries.htm
Once you've got the grant of probate (or of letters of administration), you'll be able to sell the house.