Simply pop in a Ubuntu live CD/USB and boot from it!
Firstly head over to the Ubuntu download page here, and download the 32bit .iso
Once you have done that open up "Start up disk creator" in Ubuntu and grab a blank CD/DVD/USB and choose that along with the .iso you have just downloaded. note: some computer's won't boot from USB if they are quite old so if you can use a CD/DVD.
Once the process has completed reboot your computer with the CD/DVD/USB in the computer and boot into it. This can be done my either pressing F12 on most computers or entering the bios to change the boot order, here put you CD/DVD drive at the top if you have burned to a CD/DVD or "removable storage" if you have burned to a USB.
Once booted you will get an option to "install Ubuntu", from here choose the configuration you want, e.g. Erase Windows and reinstall, Install alongside Windows etc.
Once this process has completed you will have a brand new working Ubuntu 12.04 installation.
If you need anymore help let me know.