Maybe when they spawn make it so they have to walk on a pressure plate, and use variable triggers to run the command /rules?
They have heaps of tuts, just keep trying till you get it. It would be better too because they u don't need to have the plugin updates yourself variable triggers was developed to help people who can't code.
I could try to do this... AndyMcB1 Here i am finished any other features you want please comment there http://dev.bukkit.org/server-mods/firstjoincommand/ EDIT by Moderator: merged posts, please use the edit button instead of double posting.
This plugin only does it once when you join but i can add configs for other commands for every join after or more commands.