You need a modeling package capable of animation for characters or guns, some image editing app for textures, conversion tools for segments and entities are available on the main site of TGC and there is a stickie in the model and media forum on the matter.
Which package you choose depends on your budget and experience: from Milkshape to 3dStudio max all is possible. I do mostly static props so I happily do the most of my work with SketchUp.
Weapon editing as well as everything else is a matter of editing files in a decent text editor, notepad++ comes to mind.
PS Welcome to the forums(peeks at join date) and enjoy your stay
I am Jacks complete lack of surprise.
