First of all , if you are still new to this you need to try it out first before fully switching. Try wubi before fully installing , it allows you to install linux in dualboot as a regular application , that can be removed via control panel. IF you have already decided to fully switch then you need to do this:
Download the iso from ubuntu.com
Buy a DVD disc or if your computer is fairly new 2-4 years old get a 1+ gb USB drive.
Get a disc burning program like Nero or my personal favorite "Active iso burner" and make sure your computer has a disc writer ( skip this step if you plan on using a usb)
- in case you plan on using a usb , get unetbootin or a simillar program and burn the image on the usb(usb sticks are erasable so you won't waste it you can use any stick you have)
Stick the disc in the drive or the usb in the slot and reboot.
Go in the bios menu by hitting the delete key or one of the F keys ( the key to press is shown at POST something like "Press the X key to enter bioss setup" )
Go to the boot options menu ( it's not named that in some cases might be boot order or just boot)
Set the boot order to boot first either from the USB or CD.
Reboot and let the linux live CD boot
Start setup and follow the instructions
*IF there is a driver problem with some devices ask here or google it