Where should i start?

I am a veteran Shaiya player and over the years i have played on various servers, especially spanish speaking ones. Now with time i've become interested in starting to develop plugins (im good with programming) and i wanted to ask the most experienced players about it. I remain open to learning, thanks in advance

If with 'Plugins' you mean Scripts / Server Addons, Programming itself isnt enough.
You should learn how to use Cheat Engine and how to find addresses using it.

If you are good with that and you are able to find Addresses which are useful to
you, you can learn Reverse Engineering next. If you add this two Skills, you can
already create more than you can think of most likely. There is a ton of Cheat
Engine and Gamehacking tutorials.

Those will teach you useful Skills for Game editing in general as well.