I intend to sell my laptop and therefor I am looking for some solution to my problem. How to get rid of all date in a secure manner? And as well the files which were deleted in the past are throughout some software still accessible. My disc has couple partitions. I don't want others to read my emails, see my photos etc. What is the best way?
Personally I would boot a live CD and wipe the hard drive with scrub
See man scrub
You could use other tools such as dd
Or DBAN - http://www.dban.org/
Then re-install Ubuntu. If you wish you can perform a OEM install
SHIFT + DELETE erase the file without using Trash, from command line: shred file