Ok, for starters you'll need a couple of tools. They're fairly intuitive, but if you have questions on using them just let me know, as I've used them quite a bit.
To get models into a format that the game engine uses, you need to import the mesh into OpenBRF
There you can add meshes, assign materials and textures and do other cool things. This makes you model game-ready. Be sure to add the textures by converting them to .dds file format, and placing them in your Mount and Blade/Modules/ whateveryourmoduleis /Textures folder. Save your new BRF to your module's Resource Folder
Next, open up a file called module.ini file, then scroll down to the end of the 'load_resource = ' lines and add another line 'load_mod_resource = yourfilenamehere'
To get the item to show up in shops, you can edit the itemkinds.txt directly, but it is easier if you use a tool. My favorite for this is Morgh's Editor
. Go to the item section and add a new item, first by selecting a similiar item as a template. Once the editor generates a new entry, you can edit the stats and flags for the weapon. Be sure to check the 'merchandise' flag so it shows up in shops.
That's the basics. Let me know if I left anything out. Note that if you add an item and use an existing save, it will take a week or so gametime for it to start showing up in shops