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: purelook HD 6.1
Or adam..hmm...devil kernel,atm..boaffla...damit,i don't know