Click the BLOG menu on your home page. This takes you to your blog page with all your posts.
Click Add Page, on the upper left.
Pick Blog Entry as a page type.
Give it a name and hit return, or click the add page button at the bottom of page. You don't need to worry about all these other fields yet, you can always change them later.
You're now on your new blog post page, in edit mode. Start filling the blocks and get things looking right. Add copy to the top box [add to Main] with the intro to your post. If it's short, put it all in here. Use the second box [add to Blog Post More] for the rest of your post. What you enter in the top box shows up as the summary on your blog page.