To remove content you always start with the actual articles. That's because you cannot remove containers (categories and sections) as long as they're not empty.
2. In the # Display drop-down menu at the bottom of the screen, select all. Now all of the articles are displayed in one list.
3. Select the checkbox at the top of the list (just to the right-hand side of #). This way all of the items in the column are selected:
4. Click on the Trash button in the toolbar. Joomla! shows a message confirming that the articles have been sent to the Trash.
5. When articles are in the Trash, they're still in Joomla!'s database should you wish to restore them. However, you won't be needing these articles again. To permanently delete them navigate to Content | Article Trash. Select all of the items and click on Delete.
6. In the next screen, click on Delete again to confirm you want to delete all of the articles...