While logged into your user account and viewing most pages on the site, you can click "Blog" in the top bar to view your personal site blog. Your blog has a strip at the top of the page that allows you to access options pertaining to the blog. Click the "Submit" link and you'll be taken to a form where you can enter a new post.
There are several main fields on this page. You need to provide a title for your post, a summary of 140 or fewer characters and the actual body of the post. You are also strongly encouraged to provide the URL for an image that should accompany your post, which you can host on the site. Finally, leave the box checked if you would like your post to be promoted around the site, or uncheck it if you would rather not gain all that attention (your post will still be visible, but it won't be promoted). Click the "Preview" button when you're happy with everything, take a look at how the image will appear on the site and click "Post" to submit it.