Easy! Click the folder you want to delete and drag it to the Trash folder.
You can also do this:
Right-click the folder.
Click Delete Folder from the menu.
Confirm that you really do want to get rid of the folder by clicking Delete in the dialog box.
And there's even a third way:
Click the folder.
Click the Delete Folder icon above the folder list on the left pane.
Confirm that you really do want to get rid of the folder by clicking Delete Folder in the dialog box.
Your deleted folder is now a subfolder inside the Trash folder. If you click Trash, you'll see it.