All of your work is saved as you go, so there is no need to do anything special to save your site. It is saved for you automatically. You can just close out of the editor and come back when you're ready -- all of the work you previously created will be there waiting for you.
There is one exception where this is not true and that is if you're working on a blog post. To save a blog post to work on later (before Publishing it), be sure to click the Save to Draft button at the top of the post.