It's not clear to me whether you are planning to move entirely to Ubuntu and never use Windows on this computer again, or if you'd like to add Ubuntu to the computer so you can select either at boot. The first step is the same in either case: perform a backup of all your user files on your now Windows-only computer just like you do every week. (You do backup all your user files every week, right?)
If you want to nuke Windows and move to Ubuntu exclusively, you can install Ubuntu over the top of Windows by making that choice when you install Ubuntu, then copy your user files back onto your hard drive in the correct places (pictures to ~/Pictures, Music to ~/Music, etc.). This is pretty easy.
If you want to have access to Windows and Ubuntu, you'll need to "Dual Boot". There are many good tutorials on this, so I won't write another. The basic steps are to use the Windows disk utility to resize your Windows partition to make room for Ubuntu, then use an Ubuntu Live USB to install Ubuntu into that space and install a boot loader that gives you the option to boot into Windows or Ubuntu. If you choose the dual boot option, I wouldn't access data from the Windows side directly from Ubuntu, nor would I access the Ubuntu data from Windows. You can, but you can also screw things up and make one or the other unbootable. I'd just copy my user data from my Windows backup into my Ubuntu user space exactly as If I were nuking Windows.
