Is this the blog in question? http://alifewithbipolardisorder.wordpress.com/
If it is then by default the front page of the blog is the only page that will display all published posts on it. As soon as you publish a post not a page the error message will be gone. http://en.support.wordpress.com/post-vs-page/
For creating a post see here http://en.support.wordpress.com/posts/#publish-a-new-post
For creating a page see here
For editing see:
On the bottom of your Admin page you will find a link to the step by step Learn WordPress.com blogging Tutorial prepared by Staff. http://learn.wordpress.com/
The support documentation is all found at the Support link http://en.support.wordpress.com which is also on the bottom of your Admin page.
You can quickly and easily change themes to another one found here http://wordpress.com/themes/ and no data will be lost or negatively affected by the change. The only time you may have to do some work is if you are changing to a dramatically different theme and have done any CSS editing.
Provided you are logged in as Admin under the same username account that registered the blog go to Dashboard > Appearance > Themes and browse themes until you find one you like and click "activate" or type in the name of the theme you want to use and
click the "activate" link when it appears.
After you change your theme all you need to do is go here Dashboard > Appearance > Widgets > Inactive Widgets and re-install them where you want them to appear. The widget contents and settings will be the same as they were prior to changing themes.