Shirley,
I started with an extensive old family tree that was in my family (he had done research for many years) and then another family member (still alive has been researching my grandmother's family line) Then I started looking on line to find information and comparing it with what my family trees had in them. When two or more were the same, I was pretty sure they were right. Also, I had some old relatives that belonged to the Somerset County PA Historical Society, and they did a wonderful job of documenting information. I have researched many history books myself and researched cemetery records, etc. Sometimes, I have even called family members that I could locate phone numbers or addresses. Another way is through this site. Many relatives have contacted me when they find their family while searching through the web. That is why I make my entries public. I was thrilled when a gentleman was able to find his real mother just by finding her name and information that I had put into Geni.
Sometimes I help people find other members of their family. They start out with a relative in common and if they want, I research some of their family while I look for more of mine.