The best way to customize DarkRP is to create modules. This way you can stay off the core files and keep those updated.
It's fairly simple. The DarkRP modules directory can be found in the DarkRP modification addon at lua/darkrp_modules. If you do not know about the DarkRP modification addon then you should read this.
Making a new module
First create a folder in lua/darkrp_modules.
Put your Lua files in that folder Note: All your files must either have the sv_, the cl_ or the sh_ prefix, where sv_ stands for server (the client doesn't need them), cl_ for client (the files will be sent to the client and won't be included on the server) and sh_ for shared (the client and server will include the file)