first of all you have to get 3d modelling software. There are few that are free to download like gmax, blender or rudimentary anim8or and 2d bitmap editing program to draw textures for your object.
When you have 3d model created you may export it to msh format for orbiter and configure (define parameters and animations) using spacecraft3 module for which you only need notepad.
You can convert sc3 to dll based vessel using Artlav's spacecraft to dll converter which has good feature of creating ready zip package ready for upload on ohm.
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Bottom section here covers those topics:
http://www.orbiter-forum.com/tutorials.php