Posts are what make your blog a blog — they’re servings of content that are listed in reverse chronological order on your blog’s front page. Posts are always associated with a date, which is included in the URL. The easiest way to create new post is with the New Post button on the right side of the toolbar (it appears at the top of your screen when you’re logged in to WordPress.com).
Simply click New Post and the post editor will slide down from the top of the screen.
For each pages (except some custom templates) and posts you can define custom layout type - left sidebar, right sidebar or full width by changing it on right side when editing post / page.