well the first thing is to identify what your addon is
is it an item? a playertype? a vehicle? a server script? a client script?
you will need to spend time learning how to code or at least read code if you want to do any of these
as well as modeling, for items/vehicles/custom playertypes
the best way to get started is by opening up an addon and reading its code. just open server.cs and any other .cs files with notepad or any other text editor (not microsoft word or openoffice though)