Firstly, log in as root. Then go to directory where you want to delete file or folder.
1.To delete file, type this
2. To delete folder, type this
rm <folder> -Rvf
Note : You can delete without go to its directory where contain the folder/ file. For example, now you are in /usr folders (type pwd, stands for present work directory– I think, because i forget it =) to make you know where are you know)
Then, you want to delete file in Desktop, type this
root@computername:/usr# rm /home/username/Desktop/<file.ext>
And if you want to delete folder, type this
root@computername:/usr# rm /home/username/Desktop/<folder> -Rvf
Hope this will useful for you!