This is a question that is often asked in #fluxbox. This is pretty simple.
You need to edit your .xinitrc file. You can do this in one command by running:
echo "exec startfluxbox" > ~/.xinitrc
If you want a consolekit session then you need:
exec ck-launch-session dbus-launch --sh-syntax --exit-with-session startfluxbox
Which ever method you use as long as startfluxbox is in your $PATH then it should work just fine.