hi, I'm pretty much a computer noob and I'm only 14 so sorry if this question is stupid but, when you are building a computer do u have to buy different size cases depending on what you're putting into your computer?
also can you explain the reasons why some computer cases are more expensive? and do i need to buy a seperate cooler for the computer case or does it come with?

There are MANY different kind of form factors..the most common being ATX. Just make sure the motherboard, and power supply are both ATX, they may also be "BTX" compatible, but don't worry about that. Just and FYI, there is also BTX, Micro ATX,picoBTX,microBTX, and my personal favorite BOX.

As for part two of your question:
Some cases will be more expensive than others because of cool features for additional benefits.

For example, a more expensive case will have more fan slots, screw-less drive mounting, a removable motherboard tray, or a built-in fan controller.

A cheaper case will just have the basics, with none of the extra features like those listed above listed above.

Case pricing can also depend on the brand. Brands that make excellent cases are ones like Antec or Thermaltake, to name a few. Generic-brand cases will cost way less, though the quality and features of the generic cases will also be less.

Many cases do include case fans with it. However, the cheaper ones will include no case fans, and it will be up to you to purchase cooling gear and install it. Whether or not a case includes cooling will be explained in the specifications listing of any case you look at.

Form Factor is used to describe the physical size of the motherboard AND the pattern of the holes to mount it in the case. If you purchase an ATX case, an ATX or MATX (MicroATX) will absolutely fit the case, since MATX is 2/3 the size of ATX and uses 2/3 of the same spaced mounting holes. BTX specifications are used mostly for lay down type computers, not the stand up type desktop.
An educated guess would be about 80+% of computers are built using an "ATX Mid-Tower" case

Also, cheaper cases are generally made out of...cheaper/thinner material. Your probably thinking..Why does that matter, its a case. Well.... thats exactly what i thought until i completely bent by friends $40 case trying to get a C.D drive in (don't ask how). Anyway...in the long run you will be better off getting a more expensive, better quality case. Now, this is not true in all cases (no pun intended), just because i case may be a killer price, doesn't mean it is a bad case. Just keep your eye out for that. Also, better airflow=lower hardware temps=longer life.

If you can swing the money I would buy a Cooler Master CM Stacker. That's what I own and it has TONS of room. It has the option of mounting the power supply on the bottom or top. I choose the bottom so the CPU heat can be funneled out the top. Also mounting fans facing rear funnels heat out.

He's 14, where's he gonna be getting money from?

If you have to come here to ask for advice on cases, I'd say you should just stick with a normal case. Something with no fancy bells of whistles. You probably don't dig into your case very often, and therefore would not benefit from alot of the features expensive cases offer.

Also stay away from weirdly shaped cases, especially shoebox-shaped cases. Those probably house m-ATX motherboards, and unless you specifically shopped for one, you probably will be using a full sized ATX motherboard, which won't fit those cases.

My advice is: if you don't really know what you're messing around here, stay basic. If you want to dwelve more, we're more than happy to help. A list of components, and a budget would be good.

I'm waiting for an "I just bought a such and such, what do I do next" show of sincerity from Jandro, before I resort to too much heavy typing

I bought my first computer when i was 14..and no, it wasn't my parents money.

I went on maintenance type jobs with various different people, and earned nearly $2000, in which i bought a $1100 computer with a 19" Sony LCD (was about $600 back then)

So yeah, young people aren't completely worthless.

Haha, was thinking the exact thing..but then again that is what this website is for.

Hey man, I bought an Apple IIc when I was 14 too, yard-mowing money. I STILL own it!

Not bad....

But are you willing to spend 250 dollars on something that doesn't really do anything? I suppose you can justify a 500 dollar graphics card, but a 250 dollar case?