First of all take a look at the RunUo\docs\ folder. You will find lots of information there about commands, and items/NPCs you can add.
You can also use the [Add command in game to bring up a menu that will help you find items, or use [Add *something* to perform a search.
If you want a helper program *shameless promotion* try Pandora's Box at
http://arya.runuo.com/