There are quite a few good sites worth considering. I started on Genes Reunited, where there is a great 'community' and I met many 'relatives' that way, all researching one or other branch of my family. But having decided Ancestry.co.uk possibly had the largest amount of relevant records available, I relocated my tree over there, which you can do using a Gedcom file.
Set yourself a budget limit. There will be quite a few seemingly dead-ends, where purchasing a BMD certificate seems to be the only solution to reveal a mystery parent. However if you're lucky, someone may have already purchased it, so its worth fishing round before getting the cheque book out too often.
While its fantastic to be able to pick an aged relative's memory banks, be careful- our great-grandparents would be good at witholding sensitive family information, particularly if there was illegitimacy or adoption involved. Even census records can't be taken as gospel. They are the transcribers interpretation of our forbear's often poor handwriting, so ages will not always be accurate, and unusual surnames will be frequently misspelled.
My final hint would be that family research can be very addictive, and in theory there is no limit to the amount of evidence you can gather, if you're patient. Be prepared to find nothing for months on end, then suddenly new information will appear, and you are off again! Good luck.
