It would take forever to explain it in one post.
First, you need a 3D Modeling program
I recommend Blender, since it has all the things you need to export as a .mesh
With Blender you need a lot of extra downloads of scripts, and OgreCommandLineTools and all that.
Then you have to model your item, then export it after you've downloaded all the scripts you need, then run OgreXMLConverter on it, then put it in the game, then add it into the items.cfg. Basically, it's not easy, and can't be explained all that well as just,
This is how you make an item.
It takes practice to even get your items to look like anything in the 3D modeling program.
But if you still want to try,
more of it is explained here;
http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=11025.0