How'd you end up with the computer you have? Normally if you buy it from a retail channel (e.g. JB Hifi) it comes with the OS, etc already installed? If you went through something like a computer market or a wholesaler then that's a whole other story (which isn't really meant for amateurs)!
Anyway, tell us more about your computer and what you've got? e.g. do you have a brand new in a box laptop, do you have the Windows 8 install discs, do you have a DVD drive, is the PC already formatted with an OS, etc...