Hi guys.. i have a question...
wich linux operating system already has the kernel source so i dont have to download it?

None.

quite many do. for example the "old" RedHat (and nowadays Fedora Core) had installation discs that had "SRPM" in their name - source rpm. so one could install the apps' sources from those discs. and I guess the "biggest" distros today provide the same thing, so just try them out - like the Fedora Core does. but I don't know if you get the sources if you, for example, buy the distro and not download (and if you download, it doesn't matter which distro you choose since downloading the sources is just the same matter than downloading a source-containing install disc) - I'm not sure whether or not the salesmen put the source discs in the package too.

but you could try searching either google or the distro's website (RedHat, Fedora, SuSE etc.) for package listings...that way you can find the sources. but I'd say it would be the wisest to download the sources themselves - it's some ten megs, but it's not that much..and you get the latest sources and not some year old

oke...so now i have downloaded kernel-source-2.4.18.SuSE-313.i386.rpm.... and place it in /home/Trac3R/Desktop
but i still dont know how to install it ( im a linux newbie )

Most of them... All of the major ones usually do, like deb or slack or mdk or FC2, etc... Not the silly ones like linspire perhaps...

But even so downloading the kernel source is not a big deal, esp if you download kernel.org kernels... And sometimes you want the very latest kernel, which the distros do not have.

For example you download 2.6.8 This is a 35meg download, sure, but once you have it you do not need to download the entire 2.6.9 kernel, rather you just download the 2.6.9 patch which may be only a few meg and apply that to the 2.6.8 kernel source you already have. PLus you can often get the kernel source on magazine CD cover which saves the big download, then you can just patch it to the latest kernel.

Patching is surprisngly easy if you have not tried, and it is extremely handy if you just need a small patch to fix some problem particular to your system or hardware that becomes available...

Remember that with some distros that a kernel from kernel.org can be 'better' and more reliable than the kernel that came with the distro... My mdk 10 comes to mind

Okay, your a linux newbie... Is there a particular reason you want the kernel source... Like to install the nvidia driver?