Not a stupid question at all.
There are several things to take into consideration when deciding upon which case to select. First and foremost is going to be the form-factor. The case has to have the correct alignment of standoffs to match your motherboard. Various form-factors include (Mini-ITX, Micro-ATX, Full-ATX (or just ATX), E-ATX, BTX, and so on). These form-factors are industry standards, match the form-factors of all motherboards. Some cases will accept only one form-factor, while others could accept multiple form-factors. If you have an ATX motherboard, you'll want a case that accepts an ATX form-factor motherboard.
Next is determining your system usage. System usage will normally define what components (and their sizes) will be installed within the case. You have to make sure the case you select will be able to house all of the components (power supply, hard drives, optical drives, expansion cards) within.
Additionally, system usage will also play a determining factor in how much system cooling you will require. Will you be over-clocking? Will your be water-cooling? Does the case have sufficient install points for ample air-flow through the case/heat sinks/radiators?
Finally, there's physical size. Where is the computer going to be located once the build is complete and you're using it? On your desktop? Under your desktop? In the living room home theater cabinet? Where ever the computer is going to be located, is there ample space for the size of case you select? Is there sufficient space around the case to allow for hot air to escape and cool air to enter in?
Lastly, there's budget, construction materials, and aesthetics. How much can you spend? Does it need to be sturdy (are you going to move it around a lot)? How pretty do you want/need it to be?
-Wolf sends