Use the step by step learn wordpress.com tutorial that linked to the bottom of your Admin page at http://learn.wordpress.com
1. default sidebar display (widgets)
All widgets are both installed and removed here > Appearance > Widgets. Every theme has a default sidebar display that appears to contain actual widgets but doesn't. When you select widgets you want to use one by one, click to open, configure, click "save" and "click" close on each you override the default display. If you do not want any widgets at all then place a text widget at the top of the sidebar, click to open, enter no title or body text, then click "save" and click "close".
2. front page 404 (page not found) error
That error message can easily be addressed. Publish a post and it will be gone. By default the front page of your blog displays all posts (not pages) in reverse chronological order, with the most recently published post on top. The 404 (page not found) error message on the front page of the blog appeared because the Admin of the blog deleted the placeholder Hello World post. http://en.support.wordpress.com/not-found/