Addons For Garry's mod

I am what i guess you could call experienced Lua coder, and i want to make some addons for Garry’s mod. I do not know where to start, how to make the vtf’s and all. As well as the models. Does anyone know any good software for making the models? And how would i make the vtf’s and all of the file for the addon?

VTFs can made/edited on Photoshop, if you find a plug in for it. Should take you, like, two seconds on Google to get it. Models need an external program, I think? Don’t know a whole lot about that.