Your best bet is probably to install the kernel-headers package. To do that, you have to know what kernel you have. An easy way to do this is to type:
$ uname -r
After that, get the kernel headers package for your kernel using apt-get. Use this command:
$ apt-get install kernel-headers-2.6.8-2-386
Replace 2.6.8-2-386 with whatever your kernel version was.
Another way to get the kernel-headers is to use
Synaptic. It is a front-end for apt-get.
If that doesn't work, try getting the kernel-source package. You would do that the same way, but take off -2-386 in the kernel release.
I'm knew to Debian, so I can't really give any other advice than that. Hope that helps.