Hi there. To empty your site content, you can Bulk delete the posts and pages. Go to Dashboard -> Posts -> All Posts, and click the checkbox at the very top of the list (above the first entry, not the one next to it.)
Then, select the box that says Bulk Actions, and select Move to Trash, and click Apply. You may have to do this more than once if you have a lot of posts.
Then do the same for Pages, Comments, and anything else you wish to delete.