There are a lot of computer case manufacturers, so some cases open differently than others. Most of the time though you just need to find the screws that keep the case on, unscrew them, then SLIDE it off. If you have removed the screws (or there are not any that you can see) and you still can't get the case off, try looking for some latches near the top bottom or back to unlatch them. Also, sometimes the front cover that houses the CD or DVD drive might need to be removed before the side cover screws or latches can be seen.