This varies from car-to-car. Some cars will already have fixing points on the roof, the roof bar instructions usually tell you if they don't.
If your car has a plain roof the roof bars usually clamp around the door frames. These bars have been designed to fit in a particular position on the roof, the roof bar (or fitting kit) instructions will show you exactly where to fit the bars. Most of our roof bars also have the instructions on the 'fitting instructions' tab on the product pages.
If you have roof rails (either flush rails or traditional roof rails) you usually have more flexibility as to where you position your bars, unless the rails also have fixing points in onto which the bars attach. It's usually recommended that you position the bars around 70cm apart, fitting them far enough forwards so you can fully open your hatchback with gear on the roof. Again, the roof bar or fitting kit instructions will confirm this.