Here’s some steps:
Download and install the Ubuntu .iso file here.
Once the .iso file has been downloaded, use that file to create a bootable CD.
Once the CD has been created or if you already have an Ubuntu CD, start the computer with the CD in the computer.
Select “Start or install Ubuntu”, which after 1-2 minutes should load into the Ubuntu desktop allowing you to see what Ubuntu is like before installing it. If you do not get into this screen make sure your computer meets the system requirements to run Ubuntu.
If you wish to install Ubuntu onto your computer, double-click the Install icon on the Ubuntu desktop.
In the Install window, select the language you wish to use and click the Forward button.
Select one of the available cities in your time zone and click Forward.
Select keyboard layout and click Forward.
Next the partitioner will start. If you wish to leave space available for other operating systems, move the slider to how much of the disk you wish Ubuntu to use. Otherwise, select the “Guided -use entire disk.” Once you’ve specified the partition size you wish to use, click Forward.
If your computer has another operating system, you’ll next be prompted to migrate any of the existing data.
Fill out personal information and click forward.
Finally, click the Install button to start the install. Depending on the speed of your computer this step will take around 30-45 minutes to complete.
Click the “Restart now” button and once the computer starts to reboot make sure to remove the CD from your computer.
After the computer has finished restarting, it should boot into Ubuntu. If you have your network connection connected to the computer, it will also prompt you to download any updates that may be available.