My 2.6.8 kernel can't support usb ,It is said that one can add the usb support to kernel by compiling the usb module without rebuild the it .But I not familar with C programing under linux ,anyone can tell me the exact step to do it?thanks!

Is there a specific usb function you are looking for. USB mass storage, usb printers, usb card readers, HID devices, serial -> usb adapters......
Is there an error you are getting that you didn't post?

Following how I compiled a single module before, you would

cd /usr/src/linux-2.6.8/drivers/usb

read the README and look at the Makefile. If you don't understand it, just look for sight review and post questions

make
make install


I'm running 2.6.9 and the Makefile in my directory would create all the selected drivers that were chosen in menuconfig, so there may be more detail involved that what I've posted.
Give us a little more information please