I'm a newbie. I have recently installed Ubuntu 14.04 on my Dell Vostro 3446.
I want to partition my storage so that after installing a different OS (if I ever need to do it), I don't lose data, similar to a separate partition in Windows for data files (C: contains the OS files, and D: contains data).

I can't find any satisfactory/clear description on google search on how to do this. I have already read a lot of articles which describe how to do partitioning during installation but i want to partition after installing Ubuntu. A detailed description for this will be greatly appreciated.

Thank you.

Edited Update: Screenshot added without output to questions in the comments.

edited update