Disk management of Ubuntu under VM

Disk management of Ubuntu under VM-general Linux technology-Linux technology and application information. The following is a detailed description. Thanks to my preference for Ubuntu, I installed a lot of Ubuntu software and found that the space was insufficient.
Therefore, the space is added to 20 GB, but the system still shows that the disk is only 8 GB, and the remaining 12 GB is empty,
Root @ ubuntu :~ # Fdisk-l/dev/sda

Disk/dev/sda: 21.4 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders
Units = cylinders of 16065*512 = 8225280 bytes
Disk identifier: 0x000a7de8

Device Boot Start End Blocks Id System
/Dev/sda1*1 993 7976241 83 Linux
/Dev/sda2 994 1044 409657 + 5 Extended
/Dev/sda5 994 1044 409626 82 Linux swap/Solaris

Root @ ubuntu :~ # Df-h
Filesystem Size Used Avail Use % Mounted on
/Dev/sda1 7.5 GB 4.2G 3.0G 59%/
Tmpfs 249 M 0 249 M 0%/lib/init/rw
Varrun 249 M 312 K 249 M 1%/var/run
Varlock 249 M 0 249 M 0%/var/lock
Udev 249 M 140 K 249 M 1%/dev
Tmpfs 249 M 76 K 249 M 1%/dev/shm
Lrm 249 M 2.2 M 247 M 1%/lib/modules/2.6.28-14-generic/volat

How can I use this 12 GB space?