In the main menu, choose the "Website" option, and click on "Blog".
Click the "Add a Post" button, or "+ New Post", and use the form to write your first blog post.
Choose a category, publish date and whether or not you want the post to be immediately visable from the drop down menus. Then add tags, and SEO keywords.
When you're done click "Save Post." Make sure that you have a menu item for your blog page, so it is visible from your home page.