startfluxbox is a script which runs the file ~/.fluxbox/startup If it doesnt exist it will be generated.
startfluxbox should be started from your ~/.xinitrc if you use startx, or ~/.xsession if you run a display manager, like xdm.
The default ~/.fluxbox/startup is as follows:
# fluxbox startup-script:
# Lines starting with a # are ignored.
# Change your keymap:
# Applications you want to run with fluxbox.
# MAKE SURE THAT APPS THAT KEEP RUNNING HAVE AN & AT THE END.
# unclutter -idle 2 &
# wmnd &
# wmsmixer -w &
# idesk &
# And last but not least we start fluxbox.
# Because it is the last app you have to run it with exec before it.
# or if you want to keep a log:
# exec fluxbox -log "$fluxdir/log"
If you need to start applications after fluxbox, you can change the exec fluxbox line above to something like this:
exec fluxbox &
xsetroot -cursor_name left_ptr -fg white -bg black &
So xsetroot, ipager, and gkrellm2 will all be started after fluxbox, after giving fluxbox 1 second to startup.
For more details on what else you can do in this script, see sh(1), or the documentation for your shell.