Suppose you wanted to find out the date of birth of former Prime Minister Tony Blair. Which source would you consult?
Ideally I would try to get access to a photo ID of Tony Blair, such as a passport or driving lisense, or alternatively his birth certificate, as these could be considered primary sources. Alternatively you could ask his parents. If I were going to use second hand references, sucha s an online biography, or WikiPedia, I would make sure I check several of them to make sure that they agree with eachother.