Welcome to WikiTree, Carol.
The basic plan is for you to start adding your family tree (starting with your parents). As you move up to earlier generations, or add cousins, sooner or later you will discover ancestors and other relatives who already have profiles in WikiTree. You should not create new profiles for those people; instead you should connect the profiles to the profiles for their immediate family members (connect parent to child, for example).
You can be sure that part of your line to Thomas Jefferson is already in WikiTree. All that you need to do is connect to it, as described.
And Thomas Jefferson was already here: Jefferson-1. I'm proposing to merge your new profile for him into that existing profile.