There is a screw up in the Ubuntu installer - by default it gives root about 2.3 GB of space.
The fix could be anything from a reinstall to deleting the 30+GB partiton and growing the Ubunta root . open applications>accessories>terminal - cut and paste the output of these commands in your next post. (lowercase L at the end of fdisk)
sudo fdisk -l
1st command list mount points and space avilable
2nd command list the partition table.