There are plenty of tutorials to make addons in this and modification discussion, You can also find some scattered around the forums, It depends on what kind of addon you want to make, For instance if you wanted to make something like a item or weapon you could use modeling programs such as Milkshape, Blender, Wings3d, Sketchup, Shaper, Autodesk 3d or Maya.
Really it all depends on what you want to make, Try looking through the forums for some tutorials.
