Make your system boot from the CD.most systems do this by default if your system does not do this .Go to into your BIOS. From there go to “Boot” and go to “Boot Priorities”. Make sure CD-ROM is set to boot first and you’re all set. When you rsystem boots from the Ubuntu CD you will see this screen. select “Start or install Ubuntu”.
Now when you click forward you Ubuntu loads the disk partitioner. The partitioner seeks to resize the main partition and use the freed space, is pretty much easy to go with.And this is how it works. The partitioning would be done with the help of a slider,so if you have say 40GB harddrive sive on your system and out of the 40GB – say 10GB to Ubuntu just move the slider to the right till you see it read 30GB then can leave it at that 30GB and click forward.