The first task that faces every family historian when they begin research into an individual is to collect basic biographical details about the person under investigation. The events that are shared by everyone - birth and death - are the best place to start. In many cases marriage will also be on the list. By compiling a skeleton of facts centred on these events from legal or parish records you can then continue to flesh out other aspects of that individual's history.
'... birth and death - are the best place to start.'
We have these records because, due to massive population expansion in the 19th century, civil registration for births, marriages and deaths was introduced in England and Wales in 1837, 1855 in Scotland and 1864 in Ireland. It became a legal requirement for every birth, marriage or death to be officially registered and a certificate issued as proof.
Before this date, other records exist that contain information about these events. Usually these would be the registers of baptisms, burials and marriages that were maintained by each parish. It is important to know how to use these records to make a satisfying family tree, but you need to record the simplest information about your family first.