Nova launcher is a system app,so u need to ROOT your phone and push it to your phones system/app folder or flash a custom rom where nova launcher is already included.
If you want to root your phone,search for cf-root kernel,then find the kernel that matches your rom(eg.jvu for jvu).download odin 185 from your pc and flash the tar file in pda(odin).
But search galaxy s forum on how to do it.INSANITY rom has a very good toturial on how to operate and flash from odin
Rom: echoe-rom v2 "stripped and modified"
*~* devil2 *~*
Dual : slim "latest"