Navigate to Appearance > Customize > Body > Blog Layout to select the layout of your blog page. This page will hold all of your blog posts. You can choose from a basic frame of content+sidebar right or left, full-width (padding on left and right so the content has a set width in the center of the page), 100% full width (content will reach entire width of browser window, you can add some padding), and content + sidebar on left and right.
