Putting your family tree online seems to be the way many people start in family history. Ancestry and Genes Reunited seem to be the sites of choice in England at present. In my experience, much of the information in those trees is suspect once you look beyond the submitters’ grandparents. Newcomers often do not recognise that there may be several people who could be their great grand parents and have chosen the first they came across, correct or not. However, for the recent generations the information is likely to be a bit more accurate.
Unlike Ancestry and FindMyPast, where the index information is available on the web site for free, Genes Reunited is more restrictive and it is normally impossible to see a submitter’s tree without making contact with them, which requires subscription. However, with a little ingenuity, it is possible to extract a little more information about who submitted information about a person. This can be built up into a list from which you can determine that there is a relationship between several persons. In due course I will resubscribe to GR and try to contact the submitters to check whether my deductions are correct.
Andy Micklethwaite's Family History Blog 