I can´t seem to be able to make a simple "About me"-presentation that will be available and visible on the sidebar of the blog at all times. I made this on the Dashboard under "pages" but it doesn´t seem to be visible on the blog even though I published it.

Pages do not appear in sidebars. If you want the information you placed on your About page to appear in the sidebar then edit the Page and switch to the HTML editor. Copy the code. Go here > Appearance > Widgets. Select a text widgte and paste the code into a text widget which will display in the sidbar.

Alternatively you can use a gravatar widget. The Gravatar Widget allows you to display a Gravatar image along with some custom text. This is ideal for displaying a brief About Me section on your blog sidebar.

You can easily delete all blog content by yourself rather than waiting for Staff to do it for you.
bulk delete images & media http://en.support.wordpress.com/media/deleting-files/#bulk-deletion
bulk delete posts > http://en.support.wordpress.com/posts/edit-posts-screen/#bulk-edit
bulk delete pages > http://en.support.wordpress.com/pages/edit-pages-screen/#bulk-edit
Here's how to permanently delete Trash >
http://en.support.wordpress.com/trash/
http://en.support.wordpress.com/trash/#deleting-permanently

P.S. You will find the link to the learn wordpress.com step by step tutorial prepared by Staff on the bottom of your Admin page.