New Computer, Old Game

[master321]

I'm still playing both Diablo & Hellfire and gotta say that thought played through x times it's always fun to give it another try.. sometimes with a different / harder variant.
It's a pity that ppl dont create such games anymore, rather focuses their view on better graphics in-games than gameplay..

Anyway, what I wanted to ask is, what (in general) do you require to create your own more or less "mod"?
I know that it has sth to do with programming, hexing n stuff but specifically ??
I've always wanted well, lets say my "own" mod for this beautiful game but never knew where to start.
I'm not speaking about using tools like the Ez Modmaker, though I believe that it's great I rather want to do it from scratch if you know what I mean.
Perhaps someone could give me some tips.. I really wanna learn.

Thanks in advance.