How do I decide which PC case is right for me?

The most important consideration is that the case has room for the build you're looking to use. Your amazing graphics card will go to waste if your case doesn't have room for it inside. And of course, make sure it's designed for your motherboard form factor (ATX, AT, Micro ATX).

My system runs loudly. Are there cases that offer sound-proofing?

Yes. If you have a loud system, upgrading your case alone might solve that problem. And if you work late into the night in your bedroom, your partner may appreciate it as well. Look for rubberized cases to help solve this issue. A case with silicone grommets can also reduce the noise caused by hard drive and fan vibrations.

I'm not particularly handy. Are there easy-to-assemble models?

Some cases come with a screw-less design, or thumbless screws, which minimize the tools required and can make assembly simpler.