Does anyone have a good tutorial on how to code a Garry's Mod addon?

Id look into Codeblue maybe. My way of learning how to make addons was basically just reading code of addons coded PROPERLY.

What I mean by a PROPERLY coded addon is neat code that is properly indented+commented so you get used to good habits while your still new to code.

Honestly, it really depends on what you want to make (ULX Commands, Autorun Code, DERMA, Net Library, Modules). This question is too broad for anyone to give a precise answer