It sound sto me like you're just having trouble finding the .xinitrc file. Just type (as root or any user)
Do you see an .xinitrc file now? If not, you don't have one. Youll have to make one. One way is (from your home directory, which you are already in if you followed the above commands)
that'll start up the vi editor for a new file named .xinitrc. If you don't like vi, use pico or gedit, or kate, or whatever.
That should do what you are looking for. Otherwise, I believe you can also create a file named /etc/X11/xinitrc/xinitrc.pekwm. Then use the file /etc/X11/xinitrc/xinitrc.fluxbox as aguide for what you should put in the file. Either way should work out for you.