Where to start...
Recently I've taken interest in developing a few mods of my own. Before you ask I'm very comfortable when it comes to C and thus C++ along with a handful of other languages. It would be very helpful if someone could link the resources/tools needed. Then possibly a brief description to getting started when developing GTA 5 mods nowadays.
Thank you for the help in advanced!
EDIT: I forgot to mention this post is geared towards the PC version.
@OrangeSR Most script mods are created using ScriptHookV by Alexander Blade. It is basically just an external hook that lets you call the games script functions. More info here: http://www.dev-c.com/gtav/scripthookv/ There is an example (NativeTrainer) included in the download which should get you going in the right direction. Also a good majority of the script (native) functions are documented by the community on NativeDB (http://www.dev-c.com/nativedb/func/info/c869fe97). That is one of the best resources you will have available while creating scripts.
@OrangeSR As CamxxCore said above, you can use the ScriptHookV API directly, OR if you want, there is a much more easier to work with solution(if you think scripting in .NET is ok for you) which is basically a wrapper around the ScriptHookV API:
You can look in the Wiki section and use the Getting Started tutorial to... you know... get started?