First of all, the easiest way to run things at startup is to add them to the file /etc/rc.local.
Another simple way is to use @reboot in your crontab. Read the cron manpage for details.
However, if you want to do things properly, in addition to adding a script to /etc/init.d you need to tell ubuntu when the script should be run and with what parameters. This is done with the command update-rc.d which creates a symlink from some of the /etc/rc* directories to your script. So, you'd need to do something like:
update-rc.d yourscriptname start 2
However, real init scripts should be able to handle a variety of command line options and otherwise integrate to the startup process. The file /etc/init.d/README has some details and further pointers.