While you can usually export a tree and put it elsewhere, in reality you may find that the site you start building your tree on is the site you stick with, so choose wisely!
Check out each family history website's tree-building capability. Can you access your tree on mobile devices? Can other people look at your tree? What are the privacy options? Can you connect with other people who share ancestors on your family tree? While you can usually export a tree and put it elsewhere, in reality you may find that the site you start building your tree on is the site you stick with, so choose wisely!