You should have your editor orientation document to work through. It will walk you through the main features of the site and the things you...
This is a text widget
you can edit the sidebar content by going to APPEARANCE > WIDGETS. You can add automatic widgets like sorted categories or you can create custom widgets using the 'text' one and just add whatever you want inside. Text, links or images:
To remove this just go to the widgets settings and edit this text box to have your own message to your visitors.