This is obviously what your computer is going to be built in. There are a huge amount of cases available and they come in all sorts of shapes, colours and sizes. A lot of it will come down to your preference but there are a few things to bear in mind.
If you are going to be building a gaming computer you will be better off going for one of the specific gaming computer cases. They are generally a lot bigger than your typical PC case; you need the extra room inside for cooling systems, particularly for the graphics cards and power supplies. I repaired a gaming computer and it had a 750 watt power supply and two high specification graphic cards linked together with huge cooling fans, the power supply and graphics cards alone produced enough heat to cook with!
There are also different sizes when it comes to tower cases, you will see them described as mini, midi and full size tower cases these have different configurations for the amount of full size drives that can be housed in the case, again it comes down to what you’re going to put in your PC. The other main configuration is the size of the motherboard, either ATX or micro ATX. Obvioulsy if you go for a micro ATX case your not going to be able to fit a full size ATX motherboard in it at any time in the future so my advice would be than unless space is at a premium get a case that will take ATX motherboards and has space for a few drives.
Another thing to bear in mind is that the expensive cases are a lot better put together and thought out, some of the cheaper ones have also got very sharp edges on the metal work of the case, again something to bear in mind if you plan to be taking it apart and frequently changing parts!
You might also want to consider what’s on the front panel. It can be quite convenient to have a few USB ports on the front of the case for plugging in USB drives, it’s certainly a lot easier than trying to feel around the back, as is having a front connection for headphones and microphones if you plan to be using them a lot.
Some cases also make routing cables around the internals of the box a lot easier by providing holes and various tying points. Even if your case doesn’t provide you with these extra options I would recommend that you fix them with tie wraps to avoid any of the looms inadvertently catching onto any of the fans. You will at best stall them and at worst burn the motors out or cause damage to the wiring! Also bear in mind that even if you think you have the cables positioned out of the way, you will probably be building the system while it’s lying down and when you stand it up they will all move about. Believe me it’s better to just fix stuff with tie wraps. Even if you have to cut them off to change things around its only going to cost you a few pence to buy some more. It’s a lot more expensive to try to replace fans and broken wiring!
One of my personal hates in tower PC’s is the noise that some of them make. I use mine to make music with and I came from using a silent, as they all used to be in the early days, Atari ST. I was amazed when I first encountered PC’s as some of them sounded more like vacuum cleaners! So, high on my list was a computer cases that would make my computer quieter, so I went for something that had acoustic panels in, as I intended to keep the case for a long time and just upgrade the innards I didn’t mind spending more on a decent case. Some of the manufactures to look out for who make particularly good cases are Corsair, Thermaltake, Cooler Master, Zalman and my personal favourite Antec.