i want to download a mac os x on my pc .... what are the requirements that i need ?

Well, are you aware that you can run Mac only on certified hardware and not ANY PC?
EDIT: Try Googling OSX86

With the original "vanilla" kernel a Core 2 duo or higher, with 1gb of RAM.
use legacy kernel if using a intel atom, pentium 4 or amd processor.
Also nvidia graphics cards are the easiest to work, but recently ati/amd ones also work well

Also it seems to work great on anything that has NON INTEL graphics unless that particular integrated graphics chip was used in an actual mac. Another thing is make sure you have SSE2 and SSE3 for best results. The later Pentium 4s had both these features and every processor (I believe) since has had them.

thanks all for the informations

From personal experience, I would like to tell you to check every little driver/kext down to things that I don't know what their names are Wrong kexts and/or incompatible hardware will cause kernel panics and those are hard to fix if you're new to Mac. And if you have no idea what kexts are or how to install them, then I advise you to use a OSx86 distro, such as iATKOS.