This is not very precise, but it doesn't need to be. The goal is to distinguish a person who was living in 1900 from someone born in 1940. Women should be recorded by their maiden name. Sometimes, I'll know a husband's last name, but not his first. Once you have recorded all the information you know, decide what you want to discover.
Some researchers concentrate on their paternal line only, following their surname back.
Those interested in medical history wish to know about collateral ancestors as well. Still others go back to an ancestor and then record all of that person's descendants. Whatever you wish to do, remember you can change your mind later, so continue to record everything you discover about a family. Talk to your parents, aunts and uncles. Online, you'll find several lists of questions and interview techniques.