Best way to start would be telling us which chassis you have and which motherboard. Also what your PSU is rated for and what (if any) GPU your currently using.
Most GPU's will fit most motherboard in most towers, but if you have a small tower it can cause problems. Assuming your motherboard is fairly modern it will accept any GPU providing the PSU is large enough and the tower fits it in.
If it is a PCSpecialist machine, all those details should be in your main PCS account, it will show you exactly what was in your machine, and you will be able to copy and paste the details so we can see what they are.
If it is not a PCS machine, then those details should've been on your order/spec sheet whereever you got it from - though admittedly some places don't really tell you everything.
If so then you will be able to find it in your account section.
As a general rule, a mid range GPU will fit inside any case/motherboard combo, just make sure the PSU is big enough. You will find a sticker on the PSU itself detailing how many watts it is rated for.