Installing Linux on laptop pre-installed with Windows 8

I purchased Dell Inspiron 15R 5520 which came pre-installed with Windows 8 64bit version, I recently upgraded to Windows 8.1 64bit.

As I'm a student, I have to do some work in Linux OS. So I asked Dell Customer Care about how can I dual boot Windows and Linux OR how can I install Linux in my laptop? But they told me that '...you can install another OS, but we don't provide support for OS other than Windows...' Now what do I do?

Can anyone tell me the exact procedure for installing Linux on my system? I'll be using Ubuntu (12.04) or OpenSUSE 64bit variants. Please note that my system is UEFI with Secure Boot enabled and I don't want to disable the Secure Boot unless it's extremely necessary.

I read some posts on Internet of installing Linux after installing Windows or before installing Windows, but I'm afraid of losing my OS (I have to download all 3.6GB of update from the Store all over again?!). Please provide legit or more trusted procedure of installing Linux.

Thanks :)