Each time you work back a generation, you double the number of direct ancestors; thus you have four grandparents, eight great-grandparents and so on.
Similarly, your parents go above you, and their parents - your grandparents - above them. Each time you work back a generation, you double the number of direct ancestors; thus you have four grandparents, eight great-grandparents and so on. Continue adding data to the tree until you've run out of information, or are unsure about how accurate it is. Your job is to fill in some of the blanks, or eradicate any questions you might have about the 'facts' you have unearthed to date.