To make a TF2 weapon, you need to know how to do 3D modelling.
I recommend either Google SketchUp or Blender, which are both free modelling programs.
I've never used SketchUp, but I hear it's very easy compared to 3DS Max or Blender.
Personally, I use Blender. It's a VERY powerful modeller, animator, and renderer. It's used for 3D movies and the like in commercial environments so that the big companies can use their own modified version of Blender (It's open source) and not spend big bucks on programs such as Softimage (XSI) or 3DS Max.
You can get Blender from here. Here's a LONG tutorial that walks you through the entirety of Blender's User Interface. Here's a Facepunch thread on how to use Blender models in Source Engine-based games.
Hope this helps!