If you have a newer, more expensive case, it may come apart differently. The manufacturers of better made cases have gone to a "screw less" design. With this design, you usually take hold of the bottom of the front bezel of the case and give it a nice solid pull. The front then pulls off. It is my experience that this usually requires a few tries and some muscle. These cases are usually pretty durable. The sides then lift and slide off as does the top. Your case, in essence, comes apart in four pieces. Other cases come apart in a similar way, but after you take the front off, the top and sides come off together.
Each case is a little different in how it comes apart. There are almost as many designs as there are companies that make them. Make it easy on yourself look over the manufacturer insert of instructions. You may find some where you don't even have to remove the front, and rather you just slide the sides off. With others, you can remove the whole motherboard mounting plate and card rack combo from the case by sliding it out the back. This is convenient for making quick changes to the system, although you still have to disconnect the various cables to get it out all the way. Whatever case style you have, remember to look it all over before you attempt to gain entry. You don't want to force it and break anything - take your time. Now that this is done, you are ready to move on.
Chuck Lunsford is an owner and developer of CCSPartner.com. He offers advice on how to design and build your own personal computer. Visit his website and learn more about choosing the right computer parts