How to know which system I need 32bit or 64bit windows?

Now days you no need to worry selecting  architect bit  os based on processor support . Its almost impossible to find a processor which only support 32bit . People some time need to choose 32bit os ,specially in Linux env because there custom old application may have 32bit lib issue etc. close your eye & choose 64bit  

It depends on a CPU either it is 32 or 64 bit

it dependence on your hardware, you have to check if you CPU support 64bit OS or not, if you have more than 2 GB of RAM then it is better to use 64bit

for greater details you can check this article 32-bit vs 64-bit OS