Getting Started

Hi everyone, I've been away from the Halo community for quite some time, and would like to start messing around with modding. Although there are many tutorials and resources out there, I am very overwhelmed and don't know where to start.

At the moment I'm more interested in being able to modify gameplay rather than making guns / levels. Can someone give me some pointers as to where to start? Also a list of the (latest) necessary and useful programs would be nice.

Any help is appreciated :)