You can use gparted software to resize another partition and add resized partition to your Ubuntu partition. Goto software center and type gparted. Most of the case the first result will be the GParted software(Free software) itself install and enjoy.
Updated: Both facts mentioned by Uri Herrera are true. But I've made some assumptions here. Continue if your system has partitions like this.
1. You have at-least 3 partitions, one for Windows, second for Ubuntu, another for your stuff.
If this is so, you can resize partitions other than Ubuntu and Windows partition using gparted but you cannot add to Ubuntu since it was mounted.
So you can do from windows itself.
Browse and download Easeus partition manager. You can do both resizing and adding to Ubuntu partition. At-least you can expand up-to 30 GB.