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When you install Ubuntu, make sure that it doesn't wipe your Windows partition and just either resizes it or creates an entirely new partition out of free space. It will automatically install GRUB bootloader, however, it will automatically boot into Ubuntu if you don't make as selection after a set time.

Ubuntu, unlike other distributions of linux, doesn't have a graphical installer(means you use the keyboard to move the cursor around and type). Its pretty straightforward, and just following the instructions on the screen will get you through it fine. I know distributions like SuSE have a graphical installer which allows you to use the mouse during installation, making it that much easier. Google for a guide on how to resize your current Windows partition and from there install Ubuntu or whatever distribution you choose onto the free space.