- download the ISO image of Ubuntu
- burn the ISO image into a CD / DVD or into a USB flash drive
- configure the BIOS of your PC to boot from the CD / DVD or from the USB flash drive, depending on what you chosen during the previous step.
Once you have configured the BIOS to boot from the media of your choice, you can restart the computer. After a while, the Ubuntu boot loader GRUB appear. Here are basically two ways to go: live or installation. The first one allow you to try the Linux OS without install it. This is the recommended option if you never tried Ubuntu beforeto. The second option is to directly install the OS on your PC.
Supposing you want to install Ubuntu, steps to successfully proceed with the installation are quite easy and well explained on the installation windows.
You should choose your country and your favourite language, set up an internet connection (it is recommended but not compulsory to go through) and, most important, you should set up an admin password.
During installation Ubuntu asks you to download additional software, such as third party software. It is not compulsory, but it can increase your user experience with the Linux operative system.
One of the hardest part is disk partitioning. That means how much space you would like to reserve for Ubuntu. Here you can wipe the original OS (usually Windows), install Ubuntu alongside Microsoft OS or partition the disk manually. If you are not an expert, avoid the third option and let Ubuntu do all the job.
When all the steps are done, installation will finish within minutes and you can start to enjoy your new OS.