Once your family tree is begun, you can add ancestors in 2 ways : manually, or automatically.
Adding Ancestors Manually
At anytime, click on the of a person in your tree. Forms will appear to add the parents of that individual. Fill in as much of the form as you can, and the parents will be added to your tree. You can also add spouses or children to the subject of the tree by clicking on '+Add a spouse' or 'Add a Child' in your tree.
Adding Ancestors Automatically
At anytime, click on the of an ancestor in your tree. After a few questions, Your Folks will let you know how many ancestors were found and let you choose which ones you want to add to your tree (this requires a Premium Services subscription).